User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4003956-20120423163831/@comment-4003956-20120423171524

Also, the large sizes of monsters is most likely due to large amounts of oxygen in the atmosphere. More oxygen and large food supplies means larger creatures.

The smaller creatures like humans and humanoid cats most likely evolved into such small creatures because being able to hide in small spaces and more easily escape is a very good means of survival. Also, they would be much less of a "good catch" when a monster is looking for a meal.