It's true. Likewise, for all their environmental posturing, the average left-coaster uses about 2x as much energy on a daily basis as the typical east-coaster. However, the entire comparison is predicated on a typical urban/suburban lifestyle: wired into the electrical and phone grid, indoor plumbing, sewage connection, automobile commuting, etc. I don't know of any comparison between a completely off-the-grid non-motorized lifestyle and a high-density urban dweller's impact. It doesn't mean they haven't been done, but they're not the usually touted comparison.

However, if such a comparison did exist, I wouldn't be at all surprised if it favored the high-density urbanite. Economies of scale and whatnot. Burning peat and crapping into the nearest stream are pretty dirty activities.