It's an argument that gets made over and over again:

Paris: density 55,673/mi^2

SF: density 17,246/mi^2

(source: wikipedia)

To expand:

All of NYC - 28,053/mi^2 Manhattan alone - 72,033/mi^2 London - 13,410/mi^2

Bear in mind that's using the legal definition of Paris which excludes many of its suburbs.

The SF definition is also excluding its suburbs (i.e. Silicon Valley).

I'm mainly just talking about the density -- many more people fit into less space than in San Francisco. And its both an option for us to build up, and build in -- we could close down some of our wider car-oriented streets and build walking/biking oriented roads.