@gareth79uk,
It is my impression that, as you say, Tokyo is the largest city (urban center) . More people there partake of the urban situation (i.e., the facilities enjoyed by "city folk"). Mexico City is also one of the largest, contiguously populated centers, but many of its people do not participate in urban-type facilities (phones, sewage pickup, newspapers, sewage drainage, etc.); they often live in shanty towns--on the edge of "the city"-- much like the rural centers from where they migrated.