Earlier quoted context omitted.
I don't know if you're thinking about area or population, but by either metric London is bigger than any city in the US, and has a much better public transport system.
the right word is "denser". American cities are built for cars, so even if you take a bus somewhere you're now walking 15 minutes between buildings. This can be solved on a small scale (build "pockets" of livability), but mass transit will have a harder time surviving without other urbanization changes
That was a very loaded word to use. The problem is many people consider dense urban neighborhoods to be unlivable..