question: Why are so many prehistoric animals just bigger versions of animals alive now?
comment: It's an interesting question, but the short answer is... we're not really sure. There are a few theories. We are 66 millions out from our last mass extinction, which isn't that long in the grand scheme of things, but there we lost some ancient lineages prone to becoming giant as they evolve. Our atmosphere is also different now re: levels of oxygen. For animals today to get big, they need to be very efficient respirators. On the other hand, the biggest animal that's ever lived is alive right now... the blue whale. Bigger than any dinosaur, that's for sure.
passages: BULLET::::- Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.
  
  In the past years, however, the number of Mesozoic fossil mammals has increased decisively; only 116 genera were known in 1979, for example, but about 310 in 2007, with an increase in quality such that "at least 18 Mesozoic mammals are represented by nearly complete skeletons".
  
  Section::::From cynodonts to crown mammals.:Mammals or mammaliaforms.
   Section::::Evolution of large body size.:In marine mammals.
   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.
   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.
   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.
   Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over . Although early members of this lineage had chimp-sized brains, about 25% as big as modern humans', there are signs of a steady increase in brain size after about . There is a long-running debate about whether "modern" humans are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide at the same time as a result of interbreeding.
  
  Section::::History of life.:Mass extinctions.
   Section::::Evolution of large body size.:In terrestrial mammals.
   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
   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.
   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
   Therian mammals took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups, and underwent an exponential increase in body size (megafauna). Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity. For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the dinosaurs.
   Pleistocene megafauna
  
  Pleistocene megafauna is the set of large animals that lived on Earth during the Pleistocene epoch and became extinct during the Quaternary extinction event. Megafauna are any animals with an adult body weight of over 44 kg (97 lbs).
  
  Section::::Paleoecology.
   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.)
   "Protoceratops" was approximately 1.8 meters (6 ft) in length and 0.6 meters (2 ft) high at the shoulder. A fully grown adult would have weighed less than 400 pounds (180 kg). Smaller specimens are estimated at . The large numbers of specimens found in high concentration suggest that "Protoceratops" lived in herds.
   BULLET::::- They were largely restricted to environments that are less likely to provide good fossils. Floodplains as the best terrestrial environments for fossilization provide few mammal fossils, because they are dominated by medium to large animals, and the mammals could not compete with archosaurs in the medium to large size range. Tracks from the Early Cretaceous of Angola show the existence of raccoon-size mammals 118 Million years ago.
  
  BULLET::::- Their delicate bones were vulnerable to being destroyed before they could be fossilized — by scavengers (including fungi and bacteria) and by being trodden on.
   From antiquity, fossils of large animals were often quoted as having lived together with the giants from the Book of Genesis: e.g. the Tannin or "great sea monsters" of Gen. 1:21. They are often described in later books of the Bible, especially by God himself in the Book of Job: e.g. Re'em in verse 39:9, Behemoth in chapter 40 and Leviathan in chapter 41. With the advent of geological mapping in the early 19th century, it became increasingly obvious that much of the fossils associated with the "secondary" (sedimentary) rock, notably large animals like ichthyosaurs, mosasaurs, pliosaurs and the various giant mammals found when excavating the Catacombs of Paris, were neither those of giant humans nor of any extant animals. The geologists of the day increasingly came to use the term "antediluvian" only for the younger strata containing fossils of animals resembling those alive today.
   Fossil teeth of "D. giganteum", from the late-Miocene Sinap Formation at the Turkish site of Kayadibi are larger than those from older localities, such as Eppelsheim, Wissberg, and Montredon, indicating a tendency for increasing size of members of the species over time. These were the biggest animals of their day, protected from both predators and rival herbivores by virtue of their huge bulk. The largest mammoths did not approach them in size until the Pleistocene.
   BULLET::::- Large animals store more fat in their bodies than do medium-sized animals and this should have allowed them to compensate for extreme seasonal fluctuations in food availability.
   Late Carboniferous and Early Permian insect orders include both extant groups, their stem groups, and a number of Paleozoic groups, now extinct. During this era, some giant dragonfly-like forms reached wingspans of , making them far larger than any living insect. This gigantism may have been due to higher atmospheric oxygen levels that allowed increased respiratory efficiency relative to today. The lack of flying vertebrates could have been another factor. Most extinct orders of insects developed during the Permian period that began around 270 million years ago. Many of the early groups became extinct during the Permian-Triassic extinction event, the largest mass extinction in the history of the Earth, around 252 million years ago.
   BULLET::::- Anomalocarids were a group of very early legless marine arthropods that included the largest predators of the Cambrian, such as "Anomalocaris". By the early Ordovician they had evolved into giant (for the time) filter feeders, apparently in response to the proliferation of plankton during the Great Ordovician Biodiversification Event. "Aegirocassis" grew to over 2 m in length.
  
  BULLET::::- Order Eurypterida
   Both marine and continental faunas were essentially modern but with many more large land mammals such as Mammoths, Mastodons, "Diprotodon", "Smilodon", tiger, lion, Aurochs, short-faced bears, giant sloths, "Gigantopithecus" and others. Isolated places such as Australia, Madagascar, New Zealand and islands in the Pacific saw the evolution of large birds and even reptiles such as the Elephant bird, moa, Haast's eagle, "Quinkana", "Megalania" and "Meiolania".
   Five million years ago brought about the first hippopotami and tree sloths. Elephants, zebras, and other grazing herbivores became more diverse. Lions, members of the genus "Canis", and other large carnivores became more diverse. The burrowing rodents, birds, kangaroos, small carnivores, and vultures increase in size. There was a decrease in the number of perissodactyl mammals, and the nimravid carnivores went extinct.
  
  The first mammoths came about 4.8 million years ago.
   BULLET::::- The largest prehistoric horse was "Equus giganteus" of North America. It was estimated to grow to more than and at the shoulders.
  
  BULLET::::- The giant tapir "Tapirus augustus" was the largest tapir ever, at about 1,100 lbs (500 kgs), 2.1 metres (6.9 ft) long and 0.9 metres (3.0 ft) tall at the shoulders to 3.5 metres (11 ft) long
  
  Section::::Vertebrates.:Mammals (Mammalia).:Carnivores (Carnivora).
   BULLET::::- Elephants are the largest living land animals. They and their relatives arose in Africa, but until recently had a nearly worldwide distribution. The African bush elephant ("Loxodonta africana") has a shoulder height of up to and weighs up to . Among recently extinct proboscideans, mammoths ("Mammuthus") were close relatives of elephants, while mastodons ("Mammut") were much more distantly related. The steppe mammoth ("M. trogontherii") is estimated to have commonly weighed around 10 tonnes, making it possibly the largest proboscid, which would make it the second largest land mammal after indricotherines.
  
  BULLET::::- Order Sirenia
  