Despite what many people think, it appears that many of our larger cities do produce higher incomes for residents, even after the cost of living is considered. Not all though, as LA and Phoenix residents have more moderate incomes, despite their large city size.
Besides Houston, it appears that there's a correlation between density and income here, which would make sense. The fact that people are moving in large numbers to cities with cheap housing (think St George and Phoenix) instead of moving to cities with high-incomes may be causing part of nation's economic malaise the past decade or so.
Note: this is personal income per-capita, or the GDP of a particular city divided by its number of residents.
Also note: cities are categorized by MSA. Admittedly, it's hard to easily see how density plays into this figure (hence the strike-through), although having seemingly un-dense St George, SLC, Boise, Ogden, Phoenix, Las Vegas, and LA towards the bottom of the list appears to align with what I've stated re: the effect of density on wages. Scatter-plot to come! In addition, large student populations of Provo and Logan affect their positioning in the above figure. If anyone has real income data by city that's broken down by age, please let me know.