There is a sweet spot of city size that maximizes the available facilities and venues while maximizing the ease of navigating the city. Some people are going to like 20 million person sized cities, but others may want more variety. The number of options you have for dining at 2 a.m. will be larger in a city of 20 million people than in aCity of half a million people. You recently taught a course at Johns Hopkins on physics and democracy. Does physics shed any light on why the US has just two parties while most other democracies have had multiple parties? I don't think you need physics actually to address this question.