But, why is size so important?

First of all, large communities attract migrants from wider areas. Secondly, large size produces the structural differentiation and occupational specialization, the rise of specialized institutions and of special interest groups. To each of these structural units are usually attached subcultures—police, doctors, students, people with certain political interest or hobbies in common.

It could be said that if one in every thousand are interested in a hobby in a town of 5000 there will be only five people, but in a city of one million there will be thousand such persons, enough to support it. The formation of the subculture itself will attract more people. It is these relationships or networks that sustain differences in lifestyles observed in the cities.