I also asked this very same question recently. The answer I got was to edit the actual city scroll, so that after the actual population you threw in a few zeros.

For example, when it says "population 24,000" you would edit the actual background so there would appear to be 2,400,000"

But again, that is a last reort kinda thing if you really wanted it..lets just hope they add realism to RTW2 (if...)