The way it works it's not actually population increasing as one normally thinks of it, but rather you have gained more citizens who are paying tax.
That's the explanation given, but it fails to account for new colonies. If I land 250M colonists on a planet, eventually I'll have enough that they'll be growing at 150M/turn (with approval at 100%). This is 5,850M pop in a 39 week timespan (9 months, even though it's generally slightly closer to 8), from a base of 1,875M with 4% growth, or 2,500M with 3% growth. This basically means each and every couple is having sextuplets, or in the latter case, somewhere in between quadruplets and quintuplets.
And then there's the fact that the Torian super ability is called Super Breeder.
I'm not sure I understand if the idea is that I'm transporting all of those people over there why they're not paying taxes when they get there, nor how I have that much population to send from my home planet, regardless of whether or not they're paying taxes. On a large map, with abundant planets, this theory leads us to the conclusion that I start out with 1-2 trillion pop on my home planet, but for some reason only 8B, or 0.4-0.8%, is actually paying any taxes.
While I would agree that it would be nice if this were more realistic, it's not something that overly concerns me, even as much of a perfectionist as I am.