Why were ancient animals bigger than their modern counterparts?

Answer

It's simple: survival of the fittest. If there were two animals of the same age and gender, but one was bigger, the bigger one would be more likely to survive; mainly because it would be stronger and weigh more. With more strength and weight, it could take down more prey in a shorter period of time, e.a. pinning its prey in a corner. As more animals became advanced, there was less need to constantly fight (either for food or survival), and, having smaller bodies would conserve heat and/or energy. The reason their bodies evolved was mainly because it became unnesacary to always be suited for battle, like to carry around a machine gun in New York City. It was simply not needed.

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