Why do many vegetarians and monks have a round face and a big belly?

I have no idea. The vast majority of monks, Indian people, or vegetarians I have ever seen have not had round faces and big bellies. I am vegetarian and I don't have either. Being vegetarian definitely has nothing to do with your observation, nothing about eating meat causes a "lean" vs "round" face. As a northern European, most Asian faces seem rounder to me as we tend to have more chiseled features. So maybe it's just an ethnic difference that has nothing to do with religion or eating habits.