They need to find refuges safe from predators as the environment cools. Large Animals Being large also has consequences. Large animals can hold heat well, but may have trouble getting rid of body heat in very hot weather. This is an important problem, because proteins break down if the body gets too hot, and the organism may die if overheated. Many animals have developed ways of getting rid of extra heat by sweating or panting.
Elephants can dissipate heat from their large, thin ears.
There is some disagreement about whether the dinosaurs were warm blooded or cold blooded, or in some intermediate stage. Being large poses problems for supporting structures in the body. Therefore you get an important increase in volume, and weight. A larger animal needs more massive bones. It will need to enlarge lung size, and increase digestive capacity to get the extra food to support a larger body.