As it's been said before most of the time it is the herbivores that shrink when living in a more isolated environment. And granted, environment does not always play a part in the size of a creature, but it can. As I said, desert animals tend to be smaller because there is not enough food in the desert. Where as many plains animals are larger because the abundance of food. Buffolos, Elephants, and giraffes would be an example. Girrafes being so large because they had to reach the top of the trees, there aren't really any large trees in the desert, so therefore no need for tall animals there.