[inglor_cz]

Lots of people can live in a single unit of land, unless local zoning rules actively forbid that.

Tokyo occupies about twice as much land as Los Angeles, but has four times as many people.

Too many American cities forbid even relatively modest density such as duplexes or small apartment buildings (with, say, 6 apartments per building).

Demanding that people in metropolitan regions build exclusively single family homes on large lots is insanely uneconomic, and, arguably, a failure of democracy at a local level.

[bobthepanda]

You also don't even have to urbanize all that much to get higher density.

Nassau County in New York is one of the birthplaces of postwar suburbia and has a population density higher than Los Angeles County, ranking at 22nd in the country. (4,705 people per square mile vs 2,420.)

[gottorf]

To be fair, LA County includes mountains (Angeles National Forest among others) and a desert and a whole lot of nothing on the other side of those mountains. Nassau County is almost all developable land.

I bet if you compared like to like, the density won't be too far off.