Ok... double the space allocated to the cities and you're still at about 1/3 of the greater Huston area.
Pave every single square foot of the nine counties that make up the Bay Area including Marin, Sonoma, and Napa... and you're still 3,000 square miles short of the greater Huston area.
My disagreement was about "a lot of land that you can't reasonably build a city on". I don't doubt your numbers.
That said, Tokyo, which is a wonderful city to live in, and on more earthquake prone land, has 14 million people on 5,194 square miles: https://en.wikipedia.org/wiki/Tokyo.
Pave every single square foot of the nine counties that make up the Bay Area including Marin, Sonoma, and Napa... and you're still 3,000 square miles short of the greater Huston area.