Dracosolon
King
One thing to keep in mind is that city size in Civ games isn't really about simulating real life population growth, but what capacity is best for gameplay. This is why the modern era doesn't see your cities explode in size to the same extent as what happened in the 20th century. If that was properly reflected and costs in the modern era would have to be truly enormous, else you'd end up with everything being completed in a few turns.