Why are creatures smaller than they were in prehistoric times?

The term "Prehistoric Times" covers a lot of pre history. I believe that what you are referring to may be the advanced size of many mammals during the last glaciation. Many mammals were larger than today. The mechanism here was genetic mutation and evolution. Animals of larger body size had an advantage in being able to more easily retain body heat. Less surface area per mass was exposed to the cold. Thus all bodily chemical reactions could occur as normal.