I'm no expert at population density, but theres a discussion on the Paizo boards regarding this issue. Remember more people lived in tighter quarters in previous times. So an apartment today that holds a family of three, might hold a dozen or more people in a different age. Thus a city like the one I live which has a population of 17,500+, in square mileage might hold 75,000 or more. Depends on your setting of course. There were more people in less space in previous eras.