Okay... According to the NCR Sheriff during FO2, The NCR capital has roughly 3 thousand people living within it and is growing. However, according to the Vault City Travel Logs, the NCR has a population in the tens of thousands. At first I was thinking 3K was way too low for the total population of the NCR, but then realized that most people in the city where born there (according to Prez Tandi's talking head, Most people in the NCR came from Vault 15).

So that raises the question as to how the NCR supposedly has such a large population by the time FO:NV happens (less then 100 years right?). I understand that the NCR grew and absorbed other "city-states", but I would still have thought that the NCR would have had the largest population of all of the cities in Core Region.

So would that mean to total population of California during FO2 would have been at most 10K-20K?

The population was stated to be 700,000 total according to NCR Census.