question: why have animals, in general, become way smaller than their giant ancestors millions of years ago?
comment: It's not clear that animals in general are getting smaller. The evolutionary postulate called [Cope's Rule]( URL_0 ) suggests the opposite, though it doesn't appear to be universal. Giant prehistoric animals are exciting to learn about, so people are disproportionately aware of them. However, you're probably familiar with some counter-examples; many natural history museums have displays of the horse evolving from smaller ancestors.
passages: 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.
   Section::::Evolution of large body size.:In marine mammals.
   Phyletic dwarfism
  
  Phyletic dwarfism is the decrease in average size of animals of a species.
  
  There are a few circumstances that often lead to species doing this.
  
  Lack of predators of smaller creatures can allow smaller members of a species to survive.
  
  The lack of resources to sustain a large population of larger animals can pick off the largest specimens.
  
  Available resources being more beneficial for smaller creatures can also do so.
   Section::::Evolution of large body size.:In terrestrial mammals.
   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
   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.
   BULLET::::- The extant capybara ("Hydrochoerus hydrochaeris") of South America, the largest living rodent, weighs up to . Several recently extinct North American forms were larger: the capybara "Neochoerus pinckneyi" (another neotropic migrant) was about 40% heavier; the giant beaver ("Castoroides ohioensis") was similar. The extinct blunt-toothed giant hutia ("Amblyrhiza inundata") of several Caribbean islands may have been larger still. However, several million years ago South America harbored much more massive rodents. "Phoberomys pattersoni", known from a nearly full skeleton, probably reached . Fragmentary remains suggest that "Josephoartigasia monesi" grew to upwards of .
  
  BULLET::::- Superorder Laurasiatheria
  
  BULLET::::- Order Carnivora
   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
   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.
   Megafauna – in the sense of the largest mammals and birds – are generally K-strategists, with high longevity, slow population growth rates, low mortality rates, and (at least for the largest) few or no natural predators capable of killing adults. These characteristics, although not exclusive to such megafauna, make them vulnerable to human overexploitation, in part because of their slow population recovery rates.
  
  Section::::Evolution of large body size.
   One observation that has been made about the evolution of larger body size is that rapid rates of increase that are often seen over relatively short time intervals are not sustainable over much longer time periods. In an examination of mammal body mass changes over time, the maximum increase possible in a given time interval was found to scale with the interval length raised to the 0.25 power. This is thought to reflect the emergence, during a trend of increasing maximum body size, of a series of anatomical, physiological, environmental, genetic and other constraints that must be overcome by evolutionary innovations before further size increases are possible. A strikingly faster rate of change was found for large decreases in body mass, such as may be associated with the phenomenon of insular dwarfism. When normalized to generation length, the maximum rate of body mass decrease was found to be over 30 times greater than the maximum rate of body mass increase for a ten-fold change.
   Megafauna
  
  In terrestrial zoology, megafauna (from Greek μέγας "megas" "large" and New Latin "fauna" "animal life") are large or giant animals. The most common thresholds used are weight over or (i.e., comparable or larger in mass than a human) or over a tonne, (i.e., having a mass comparable to or larger than an ox). The first of these include many species not popularly thought of as overly large, such as white-tailed deer and red kangaroo.
   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
   Other common uses are for giant aquatic species, especially whales, any larger wild or domesticated land animals such as larger antelope and cattle, as well as numerous dinosaurs and other extinct giant reptilians.
  
  The term is also sometimes applied to animals (usually extinct) of great size "relative to" a more common or surviving type of the animal, for example the dragonflies of the Carboniferous period.
  
  Section::::Ecological strategy.
   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.)
   BULLET::::- The largest sirenian at up to is the West Indian manatee ("Trichechus manatus"). Steller's sea cow ("Hydrodamalis gigas") was probably around five times as massive, but was exterminated by humans within 27 years of its discovery off the remote Commander Islands in 1741. In prehistoric times this sea cow also lived along the coasts of northeastern Asia and northwestern North America; it was apparently eliminated from these more accessible locations by aboriginal hunters.
  
  BULLET::::- Superorder Xenarthra
  
  BULLET::::- Order Cingulata
   As stated by J. B. S. Haldane, large animals do not look like small animals: an elephant cannot be mistaken for a mouse scaled up in size. This is due to allometric scaling: the bones of an elephant are necessarily proportionately much larger than the bones of a mouse, because they must carry proportionately higher weight. Haldane illustrates this in his seminal 1928 essay "On Being the Right Size" in referring to allegorical giants: "...consider a man 60 feet high...Giant Pope and Giant Pagan in the illustrated "Pilgrim's Progress:" ...These monsters...weighed 1000 times as much as Christian. Every square inch of a giant bone had to support 10 times the weight borne by a square inch of human bone. As the average human thigh-bone breaks under about 10 times the human weight, Pope and Pagan would have broken their thighs every time they took a step." Consequently, most animals show allometric scaling with increased size, both among species and within a species. The giant creatures seen in monster movies (e.g., Godzilla, King Kong, and Them!) are also unrealistic, given that their sheer size would force them to collapse.
   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.
   BULLET::::- The novel "The Food of the Gods and How It Came to Earth" by H. G. Wells describes a kind of food that can accelerate and extend the growth process, which when introduced to the world causes great upheavals. In Wells' novel, giants have great powers, and they seek to continue growing and improving; only the small people with their small minds stand in their way. This is a symbol of social groups with great potential suppressed by mainstream society, and an expectation for them to eventually change the world in a radical way. Though one of Wells' lesser-known works, many of the features of the novel have been incorporated into other works.
   Furthermore, the changes in the structure of human brains may be even more significant than the increase in size.
   BULLET::::- The largest of this order and family of prickly-skinned, small mammals is the greater moonrat ("Echinosorex gymnura"), native to the rainforests of the Malaysian Peninsula as well as Sumatra and Borneo. The maximum size of this species is over and . The moonrat is a member of the same family as hedgehogs, which are typically much smaller than the moonrat. Even larger was the giant gymnure "Deinogalerix" from Miocene Europe. It was estimated to grow larger than a house cat.
  
  Section::::Hyraxes (Hyracoidea).
   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:
   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.
   Section::::Possible mechanisms.:Differential extinction rates.
   Section::::Evolution of large body size.:In giant turtles.
  
  Giant tortoises were very important components of late Cenozoic faunas, occurring in virtually every continent until the arrival of homininans. The largest known terrestrial tortoise was "Megalochelys atlas", an animal that probably weighed about 1,000 kg.
  
  Some earlier aquatic Testudines, e.g. the marine "Archelon" of the Cretaceous and freshwater "Stupendemys" of the Miocene, were considerably larger, weighing more than 2,000 kg.
  
  Section::::Megafaunal mass extinctions.
  
  Section::::Megafaunal mass extinctions.:Timing and possible causes.
  
presuppositions: Animals have become way smaller than their ancestors. In general, animals evolve to smaller forms.
corrections: Animals may not have become way smaller than their ancestors. It's not certain that the majority of animals evolve to become smaller, there are many examples of them getting larger.
raw_presuppositions: Animals have become way smaller than their ancestors. In general, animals evolve to smaller forms.
raw_corrections: Animals may not have become way smaller than their ancestors. It's not certain that the majority of animals evolve to become smaller, there are many examples of them getting larger.