question: Why are creatures in general much smaller today than they were millions of years ago with the dinosaurs?
comment: Depends on the creature. Blue whales are the largest animal that has ever existed on Earth as far as we can tell, and it exists now. And mammals were tiny in the age of dinosaurs but we're until recently huge. But as far as selective pressures for body size, the dilemma is between having size as a way of being safer from predators/using size to be a better predator versus needing more resources to support more mass. In resource rich environments, the benefits of being bigger tend to outweigh the burdens of needing more food. But as food becomes scarcer, smaller body sizes tend to become more successful. And that switch happens quickly in instances of generations-long scarcity.
passages: Early in the Cenozoic, following the K-Pg event, the planet was dominated by relatively small fauna, including small mammals, birds, reptiles, and amphibians. From a geological perspective, it did not take long for mammals and birds to greatly diversify in the absence of the dinosaurs that had dominated during the Mesozoic. Some flightless birds grew larger than humans. These species are sometimes referred to as "terror birds," and were formidable predators. Mammals came to occupy almost every available niche (both marine and terrestrial), and some also grew very large, attaining sizes not seen in most of today's terrestrial mammals.
   Section::::Evolution of large body size.:In marine mammals.
   At one level, it is possible that the clade's increased vulnerability to extinction, as its members become larger, means that no taxon survives long enough for individuals to reach huge sizes. There are probably also physically imposed limits to the size of some organisms; for instance, insects must be small enough for oxygen to diffuse to all parts of their bodies, flying birds must be light enough to fly, and the length of giraffes' necks may be limited by the blood pressure it is possible for their hearts to generate. Finally, there may be a competitive element, in that changes in size are necessarily accompanied by changes in ecological niche. For example, terrestrial carnivores over 21 kg almost always prey on organisms larger, not smaller, than themselves. If such a niche is already occupied, competitive pressure may oppose the directional selection. The three Canidae clades show a trend towards larger size before becoming extinct.
   Early in the Cenozoic, following the K-Pg extinction event, most of the fauna was relatively small, and included small mammals, birds, reptiles, and amphibians. From a geological perspective, it did not take long for mammals and birds to greatly diversify in the absence of the large reptiles that had dominated during the Mesozoic. A group of avians known as the "terror birds" grew larger than the average human and were formidable predators. Mammals came to occupy almost every available niche (both marine and terrestrial), and some also grew very large, attaining sizes not seen in most of today's mammals.
   BULLET::::- The dwarfing of animals is not explained by overkill. Numerous authors, however, have pointed out that dwarfing of animals is perfectly well explained by humans selectively harvesting the largest animals, and have provided proof that even within the 20th century numerous animal populations have reduced in average size due to human hunting.
   The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest were an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as "Paraceratherium" and "Palaeoloxodon" (the largest land mammals ever) were dwarfed by the giant sauropods, and only modern whales surpass them in size. There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.
   The large size of insects and amphibians in the Carboniferous period, when the oxygen concentration in the atmosphere reached 35%, has been attributed to the limiting role of diffusion in these organisms' metabolism. But Haldane's essay points out that it would only apply to insects. However, the biological basis for this correlation is not firm, and many lines of evidence show that oxygen concentration is not size-limiting in modern insects. There is no significant correlation between atmospheric oxygen and maximum body size elsewhere in the geological record. Ecological constraints can better explain the diminutive size of post-Carboniferous dragonflies - for instance, the appearance of flying competitors such as pterosaurs, birds and bats.
   The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest were an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as "Paraceratherium" (the largest land mammal ever) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size. There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.
   Matthew Bonnan has shown that sauropod dinosaur long bones grew isometrically: that is, there was little to no change in shape as juvenile sauropods became gigantic adults. Bonnan suggested that this odd scaling pattern (most vertebrates show significant shape changes in long bones associated with increasing weight support) might be related to a stilt-walker principle (suggested by amateur scientist Jim Schmidt) in which the long legs of adult sauropods allowed them to easily cover great distances without changing their overall mechanics.
  
  Section::::Description.:Air sacs.
   The respiratory system is an important factor that limits the size of insects. As insects get larger, this type of oxygen transport is less efficient and thus the heaviest insect currently weighs less than 100 g. However, with increased atmospheric oxygen levels, as were present in the late Paleozoic, larger insects were possible, such as dragonflies with wingspans of more than two feet.
   Section::::Evolution of large body size.:In terrestrial mammals.
   Section::::Description.:Lilliput effects.
  
  The Lilliput effect is a term coined in 1993, used to describe the dramatic reduction in size observed in populations of a taxa which have survived a major extinction event, attributable to a variety of environmental factors. This effect has been documented in numerous “survivor” taxa of such events. In his work, Adam K. Huttenlocker examined fossil records of "Moschorhinus kitchingi" from before and after the mass extinction demarcating the Permo-Triassic boundary. Specifically, he looked at changes in cranial size and limb bone histology indicating vascular growth patterns.
   Section::::Vertebrates.:Birds (Aves).:Cormorants and allies (Pelecaniformes).
  
  The largest-known cormorant was the spectacled cormorant of the North Pacific ("Phalacrocorax perspicillatus"), which became extinct around 1850, was larger still, averaging around and .
  
  Section::::Vertebrates.:Birds (Aves).:Bony-toothed birds (Odontopterygiformes).
   Section::::Examples.
  
  The following are some notable examples of animals often considered as megafauna (in the sense of the "large animal" definition). This list is not intended to be exhaustive:
  
  BULLET::::- Clade Synapsida
  
  BULLET::::- Class Mammalia (phylogenetically, a clade within Therapsida; see below)
  
  BULLET::::- Infraclass Metatheria
  
  BULLET::::- Order Diprotodontia
   Later ornithopods became larger, but never rivalled the incredible size of the long-necked, long-tailed sauropods that they partially supplanted. The very largest, such as "Shantungosaurus", were as heavy as medium-sized sauropods at up to 23 metric tons (25 short tons) but never grew much beyond 15 metres (50 feet).
  
  Section::::Classification.
  
  Historically, most indeterminate ornithischian bipeds were lumped in as ornithopods. Most have since been reclassified.
  
  Section::::Classification.:Taxonomy.
   Subsequent to the Cretaceous–Paleogene extinction event that eliminated the non-avian dinosaurs about Ma (million years) ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant. Starting from just a few kg before the event, maximum size had reached ~50 kg a few million years later, and ~750 kg by the end of the Paleocene. This trend of increasing body mass appears to level off about 40 Ma ago (in the late Eocene), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude. However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of "Indricotherium" 30 Ma ago. (Since generation time scales with "body mass", increasing generation times with increasing size cause the log mass vs. time plot to curve downward from a linear fit.)
   Because of the small initial size of all mammals following the extinction of the non-avian dinosaurs, nonmammalian vertebrates had a roughly ten-million-year-long window of opportunity (during the Paleocene) for evolution of gigantism without much competition. During this interval, apex predator niches were often occupied by reptiles, such as terrestrial crocodilians (e.g. "Pristichampsus"), large snakes (e.g. "Titanoboa") or varanid lizards, or by flightless birds (e.g. "Paleopsilopterus" in South America). This is also the period when megafaunal flightless herbivorous gastornithid birds evolved in the Northern Hemisphere, while flightless paleognaths evolved to large size on Gondwanan land masses and Europe. Gastornithids and at least one lineage of flightless paleognath birds originated in Europe, both lineages dominating niches for large herbivores while mammals remained below 45 kg (in contrast with other landmasses like North America and Asia, which saw the earlier evolution of larger mammals) and were the largest European tetrapods in the Paleocene.
   BULLET::::- Foster and others observed that no other theropod inhabiting Asia or North America during the Campanian or Maastrichtian achieved a body size within "two orders of magnitude" of contemporary tyrannosaurs.~paleobio133-134~ They further speculated that this gap in body size may be attributable to juvenile tyrannosaurs occupying the ecological niches once exploited by other medium-to-large sized theropods.
  
  BULLET::::- Holtz found that within the coelurosaurs, tyrannosaurs were arctometatarsalians; meaning they were closer to ornithomimosaurs than to birds.
   Many predators are powerfully built and can catch and kill animals larger than themselves; this applies as much to small predators such as ants and shrews as to big and visibly muscular carnivores like the cougar and lion. 
  
  Section::::Specialization.:Diet and behaviour.
   About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals. Insects were particularly successful and even today make up the majority of animal species. Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from "reptile"-like lineages), mammals around 129 million years ago, homininae around 10 million years ago and modern humans around 250,000 years ago. However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.
   An extensive study published in 2015 supports the presence of a trend toward larger body size in marine animals during the Phanerozoic. However, this trend was present mainly in the Paleozoic and Cenozoic; the Mesozoic was a period of relative stasis. The trend is not attributable simply to neutral drift in body size from small ancestors, and was mainly driven by a greater rate of diversification in classes of larger mean size. A smaller component of the overall trend is due to trends of increasing size within individual families.
   BULLET::::- The largest living primate, at up to , is the gorilla ("Gorilla beringei" and "Gorilla gorilla", with three of four subspecies being critically endangered). The extinct Malagasy sloth lemur "Archaeoindris" reached a similar size, while the extinct "Gigantopithecus blacki" of Southeast Asia is believed to have been at least two times larger. Some populations of archaic "Homo" were significantly larger on average than recent "Homo sapiens"; for example, "Homo heidelbergensis" in southern Africa may have commonly reached in height, while Neanderthals were about 30% more massive.
  
  BULLET::::- Order Rodentia
   Section::::Maximum size.:Historical claims.
  
  Section::::Maximum size.:Historical claims.:Misidentifications.
   Section::::Validity.
  
  Cope recognised that clades of Cenozoic mammals appeared to originate as small individuals, and that body mass increased through a clade's history. Discussing the case of canid evolution in North America, Blaire Van Valkenburgh of UCLA and coworkers state:
   The smallest known dinosaur (excluding modern birds) is "Anchiornis", a genus of feathered dinosaur that lived in what is now China during the Late Jurassic Period 160 to 155 million years ago. Adult specimens range from long, and the weight has been estimated at up to . Nevertheless, sizes of dinosaurs are commonly labelled with a level of uncertainty, as the available material often (or even usually) is incomplete.
  
  Section::::Animals.:Dinosaurs.:Birds.
  
presuppositions: Currently living species are smaller than those living millions of years ago. Current living creatures are smaller than those living millions of years ago.
corrections: The size of creatures vary based off of what kind of animal they are and in many circumstances present animals are larger than the ones in the past.  Creatures have become larger, smaller, or have stayed the same over time. 
raw_presuppositions: Currently living species are smaller than those living millions of years ago. Current living creatures are smaller than those living millions of years ago. Currently living species are smaller than those living millions of years ago.
raw_corrections: The size of creatures vary based off of what kind of animal they are and in many circumstances present animals are larger than the ones in the past.  Creatures have become larger, smaller, or have stayed the same over time.  The largest creature ever (blue whales) are currently living.