text: Basic answer - all warm-blooded animals must expend some energy to maintain their bodies at an adequate temperature and avoid hypothermia. Now, the heat capacity of an animal is proportional to its volume, but the rate at which it loses heat to its environment is proportional to the surface area. The practical consequence is that the bigger the ratio between volume and surface area, the longer it takes the animal to lose significant amounts of heat and the less energy it has to expend on keeping warm - which in turn means, that the animal can afford to have a less active metabolism and eat less food in proportion to its body weight. Now, volume increases as the cube of the linear dimension, but surface area only increases with the square of the linear dimension, which means that large animals have a bigger volume to surface area ratio and thus tend to have less active metabolisms than smaller animals. n.b. that this only applies to warm-blooded animals. Cold-blooded animals, because they do not try to maintain constant body temperature, tend to be minimally affected by this principle.