Apparently, no matter how dense your city, it is at the cusp of becoming "too dense." Manhattanites feel this way; this New York Times article explores how many people Manhattan could hold. Manhattan's a pretty dense place, so it takes Kowloon Walled City to make the island look sparsely populated.
Matt Yglesias makes the point that California would hold over 188 million people if it were settled at New Jersey's density. This would be a good thing, he argues, because California is a nice place to live and has a temperate climate. There are problems with comparing states like this, though, because one state (such as California) might have a lot of inhospitable land.
But there is no such problem in making comparisons between cities. The NYT article made me wonder how large Austin would be if it were settled at the densities of some other American cities.
According to the 2010 census, Austin had 790,390 people on 297.9 square miles, for a (standard) density of 2,653.2 people per square mile. Obviously, Austin would be gigantic if it were populated at San Francisco's density of 17,170 ppsm (in which case Austin would have 4,327,000 residents) or at New York City's average density (in which case it would have 7,411,000 residents).
But no one considers Dallas or Houston to be terribly dense places -- they are the prototypical suburban sunbelt megalopoli. Yet both are much denser than Austin. If Austin were populated at Dallas' density, it would have an extra 258,000 people. At Houston's density, it would have an extra 289,000 people.
A lot of the cities we think of our as our peers are much denser. If Austin were settled at Denver's density, it would have an extra 363,000 people. At San Diego's density, an extra 402,000. And at Portland's density, an extra 513,000. Columbus, Ohio is probably Austin's true "peer" city - it has almost the identical population, is home to a giant state university, and is the state capital. Austin would have to add 290,000 to reach Columbus' density.
People around here would raise hell, of course, if the City Counil were to allow the kind of development needed to raise Austin's density to that of its peers. But the question is, Why? Obviously, some of the nicest places in the country are nice places despite significantly higher densities. It may even be the case that they are nice places because of significantly higher densities. I've always thought it really weird for people to complain that rising density will ruin their quality of life when so many places with an equally good or better quality of life are already much denser.
PS. There's a cool 1989 German documentary on Kowloon Walled City on Youtube. Needless to say, life at a density of 3.2 million people per square mile was pretty strange. H/t @MarketUrbanism.