Good argument. In my hometown, people are incredibly community minded, kind and generous -- as long as you fit the norm. So I wonder if it's less about population size and more about homogeneity.
In our current world, we're far more likely to run into people with different values, beliefs, etc. then ourselves, which means more conflict and often more violence. I strongly believe that diversity makes us stronger, but it is harder. I still hold out hope that we'll see the day where we learn to deal with the challenges we face without resorting to violence.
I think both. Similarly oriented people obviously get well together. Nonetheless, at some point, if too many people go after the same thing, shit is going to hit the fan.