It's a reasonable assumption that humans will eventually swarm any environment they find themselves in. More importantly, the game mechanics don't assume your population = total planet population. Your population consists of the total number of taxpaying citizens on the planet - the population growth statement that comes up at some quarterly reports days as much, as humans cannot possibly reproduce at the rates that planets grow. It's that more people are becoming citizens instead of merely living there. Given that, it's not surprizing there is an arbitrary cap on the number of citizens on any planet.
IMO this would not add anything to the game. Yes, it may be random at colonization, but it would be leveled out with farms before too long. Even the AI would probably adjust without really noticing.
Cool, I get ya. Maybe I'm just a little board with seeing 4PQ-8.00b planets. C'mon now, do you even consider droping a colony ship on a 4QP early on? No, me neither. But, if there was the possibility that planet had a 16.00b pop cap to start even with only 4 tiles.....
I'm just saying, in a type of game where we all try to break the it down as mathematically as we can, why not add a simple variable to even add more to the game. I'd mod the it myself if I could but I'm still in school just learning how to do it.