To elaborate on Martin's comment on heat loss:
A small animal must generate more heat per gram of mass to maintain the
same temperature as a large animal. This is because heat loss occurs
at the surface (skin) and the proportion of surface area to mass is
higher in small animals. Put another way, no part of a mouse is very
far from it's skin, while parts of an elephant are much farther from
the skin, so it doesn't loose heat as fast.
A related question, also unanswered, is why the life-span of an animal
generally increases with body size. The answers to these two may be
related.
-Paul Schlosser
