[nayuki]

I think you're forgetting that house foundations are enormous slabs of concrete, especially on a per-resident basis.

[quantumwannabe]

Not really, a 1000 sq ft apartment with concrete floors and ceilings has the same amount of concrete used as a two story 2000 sq ft house in an area that uses slab foundations. The house also has no steel and did not require extremely heavy equipment to build.