Sure, not all Saints supporters live in St. George. That's true of any club. But the majority of Sydney-based fans surely do live in St. George and the majority of those who don't would live broadly within the area around the locus of points equidistant from Kogarah and Homebush or Kogarah and Moore Park. Plus, of course, it's easier for Illawarra-based fans to get to Jubilee than it is to the other two grounds.
Then, of course, you have to remember the failure that was Saints playing out of Aussie Stadium 2000-2002 and the SCG 1986-1987.
In any case I think we probably need to fill the ground a few more times before we talk about it needing more capacity. In any case by the middle of next year Jubilee will have a capacity of about 22-23k which would make it more than big enough for our needs in the foreseeable future and one of the highest-capacity suburban grounds in the game.