I don't argue that we should cover all the land available with houses, I like wild grass, but I just point out that if we really hadn't enough space -- we would have settled it already.
I've never heard the now popular "Earth is overpopulated" rant from anyone who lives outside major metro. Going by train in Russia or Japan (yes, Japan) and watching empty fertile lands for hours is a really useful educational experience.
Also, 1. Americans love to live in private houses. And that takes up way more space, and energy to maintain than condos/flats. Yes, some people in other countries just cannot afford private house, but I've noticed often they just feel anything bad about living in a condo.
2. Americans like their cars. And parking/highways take a lot of space, obviously.