Why do animals in warm climates have big ears?

To expel heat more efficiently. In the case of animals such as the fennec fox, those large ears have a large amount of blood vessels close to the skin which allows heat to dissipate more quickly. In the case of animals such as elephants (particularly African elephants, who have larger ears than their Asian cousins), the ears also keep the animal cool by acting as a fan of sorts when the elephant waves them back and forth.