The energy an animal must expend to move uphill is proportional to its body weight, whereas the animal' s energy output available to perform this task is proportional to its surface area. This is the reason that small animals, like squirrels, can run up a tree trunk almost as fast as they can move on level ground, whereas large animals tend to slow down when they are moving uphill.
The assumption on which the explanation above depends is that the ratio of surface area to body weight is smaller in large animals than it is in small animals.