I noted the other day that the Census spreadsheet with density data for all U.S. metro areas had some errors and had been taken down. A corrected version has now been re-posted.
Again, do check out the nifty graphing feature that allows comparisons between cities. Here, for example, is a graph comparing the weighted densities of Portland and Austin at various distances from their repective city halls:
As you can see, Portland is much denser at the very center, Austin is much denser in the 1-2 mile band, and then Portland is generally much denser between the 2-mile and 13 mile marks. (The cities are pretty close between 5-7 miles).
Portland's MSA population-weighted density is roughly a third denser than Austin's. The chart above gives part of the reason -- Portland is denser across most distances. But this chart in conjunction with the last provides the full explanation:
Portland has a huge bulge in population between the 4-mile and 12-mile marks, where it is consistently more dense than Austin. So it's not just that Portland has a large swath that is denser than Austin -- that swath also happens to be where the bulk of the people live.