I think that it's because in larger populations, it's easier to remain anonymous and people can be real jerks when they don't think anyone will find out. The internet is a prime example.

Fortunately, people aren't ALL that way, and even those of us in larger cities encounter kind people too. But yeah, "screw your neighbor" does seem more prominent in larger cities.

Addressing one of your later comments, yes, I do think the media plays a part in it too.