There are none, by the terms you're using. “Over” means greater than the level that is good. Any time you have “too much” of something, it's normally a negative thing.
But the benefits of a very large population, there are some. Transportation becomes more efficient, many cultural institutions can flourish because of the concentration of people and wealth. More people equals higher demands which can make some things cheaper to manufacture on a mass scale. We can have a greater variety of cultures, languages, foods, types of music, etc.
hope this helps!
