Cities in the US are less dense than cities elsewhere [0] because of the large extent to which US cities are designed around cars [1] (including SF, which is #104! Even our more dense cities aren't very dense relative to the world).

[0] http://www.citymayors.com/statistics/largest-cities-density-...

[1] http://www.autolife.umd.umich.edu/Environment/E_Casestudy/E_...

Depends on how you count. If you look at the metropolitan area, it's the 7th densest.

https://en.wikipedia.org/wiki/List_of_United_States_cities_b...

that's not really a good stick to measure against.