Knowing the population of a metropolitan area in a given country allows one to estimate, with approximately 85 percent accuracy, its average income and the dimensions of its sewer system. These are the laws, they say, that automatically emerge whenever people “agglomerate,” cramming themselves into apartment buildings and subway cars. It doesn’t matter if the place is Manhattan or Manhattan, Kan.: the urban patterns remain the same...the real purpose of cities, and the reason cities keep on growing, is their ability to create massive economies of scale, just as big animals do. After analyzing the first sets of city data — the physicists began with infrastructure and consumption statistics — they concluded that cities looked a lot like elephants. In city after city, the indicators of urban “metabolism,” like the number of gas stations or the total surface area of roads, showed that when a city doubles in size, it requires an increase in resources of only 85 percent...This straightforward observation has some surprising implications. It suggests, for instance, that modern cities are the real centers of sustainability. According to the data, people who live in densely populated places require less heat in the winter and need fewer miles of asphalt per capita.People, however, do not go to cities because they are more efficient, they go because their are more social and commercial interactions. West and colleagues were able to quantify Jane Jacob's points in her famous book “The Death and Life of Great American Cities.”
...whenever a city doubles in size, every measure of economic activity, from construction spending to the amount of bank deposits, increases by approximately 15 percent per capita (It also experiences a 15 percent per capita increase in violent crimes, traffic and AIDS cases). It doesn’t matter how big the city is; the law remains the same...everything that’s related to the social network goes up by the same percentage.