Mammals expend most of their energy on the endothermic warm blooded metabolism. Both Jurassic and Cretaceous times of the giants were marked by very little to no polar ice caps and the climate was globally warm. In such a climate, keeping yourself inwardly warm is not so much of an issue. Once upon a time there was a girl who really loved dinosaurs poster. What the dinosaurs were eating was directly being translated to their sizes. During times after the dinosaurs. There were indeed some truly large mammal genera although not as large as the sauropods. Who could say what would the size of mammals 10 million years from now, if polar ice caps melt and mammals live in an environment resembling that of jurassic and cretaceous.