Big things are happening on TV Tropes! New admins, new designs, fewer ads, mobile versions, beta testing opportunities, thematic discovery engine, fun trope tools and toys, and much more - Learn how to help here and discuss here.
This trope is commonly found throughout nature for two basic reasons:
Safety: Larger creatures are less likely to be threatened by smaller creatures and therefore do not tend to treat smaller creatures as threats. There's simply less need for them to threaten or attack creatures around them. Smaller creatures, on the other hand, must worry more about being attacked. They have to be more diligent in presenting a strong front to dissuade potential predators.
Energy: It takes a lot of energy to move a large body around. Large creatures tend to move more slowly and have a slower metabolism. Smaller creatures, on the other hand, are quicker and lighter. They have more energy to engage in strenuous, aggressive behavior.
Humans are no exception. Large people tend to be less hyperactive and don't need to assert themselves physically the way smaller people do. Before any social conditioning enters into the equation, large people tend to be more confident and sedate, and therefore more gentle.