First, I think we can agree that frequent interactions with another is a necessary (if not sufficient) condition for friendship. If so, it should be obvious that density is good but size is bad.
Second, I believe our major metros—especislly since the great invertion—attract a certain sort of person less likely to foster community. Your steriotypical new gentrifier is career-focused with strong sense of individuality, moving for work, and, perhaps most importantly, fresh out of school where making friends presents very different challenges.