I'm an avid fan and player of masters of orion 2, which uses population to support the population, produce and research in conjunction with colony improvements.  In GalCiv it doesn't seem to work that way, it seems like a factory provides x pu that go towards making things and the amount of population doesn't matter in that sense.  In fact from rereading the manual, and such it seems population only potentially gives you more taxes and influence, is this accurate?  In my game play so far which has been limited I really struggled keeping people happy since I tried to maximize my population (which in Moo2 is a good thing)