It is much easier for a small animal to run uphill than for a large animal, because:
It is easier to carry a small body weight.
Smaller animals have a higher metabolic rate.
Small animals have a lower O2 requirement.
The efficiency of muscles in large animals is less than in the small animals.
Metabolic rate is inversely proportoion to the body size. So, smaller animals have a higher metabolic rate, thus have quick and more energy requiredto go up the hills.