Have you played Sim City Classic? I have two cities on that which have a population over 500,000 people, better known as a megalapolis. Its not much of a population for the new game, but you had to squeeze all the buildings together just to have a chance at a megalapolis in the old game!

I used to play it as a kid, I found a way to kind of cheat and fit more zones in. Once a building has been built go to the terrain editor and cover alittle of the building with dirt, that allows you to plop another zone/building there letting you save alittle space. I wish I still had my old computer with the original on it.