You don't get billions on a out of the way world that has only been colonized for five to ten generations and started with a five digit population. Those rare perfect Garden worlds, or national important worlds get the high starting populations and the constant immigration to put them in the billions. Put those worlds with billions are still a minority in each start nation, or at least they should be.
You are grossly underestimating both natural population growth and the amount of time in play. The only worlds that have been colonized for "five to ten generations" are the very rare "new" colony projects from the final days of the Star League. Even the "minor" Star League member worlds have been colonized for thousands of years; the Star League itself lasted ~200 years, but it was literally an overgovernment over established worlds that were highly advanced interstellar empires unto themselves.
Starting population - 1,000
Average Yearly Population Growth - 3%
After 300 years
Population - 7,098,513
As for FASAnomics keep in mind that the vast majority of even inner sphere worlds have low populations. Beyond named worlds like Solaris, sector, and nations capitals that are in the billions. The majority of the rest of those worlds have populations in the low millions, with more than a few worlds that edge the periphery only being six digit.
Plug in a more reasonable 5% growth and you have 22,739,961,286. Yes, 22 billion. The 3% LordSunhawk plugged in is what you'd expect from a fully developed world with no free land to be colonized, ie. the modern day, not anywhere with a frontier. Historically when there's land, people tend to have large families in order to obtain said land. I would hardly expect an actual 22 billion from such a world but hitting a single-digit billions in 300 years is dead easy, that'd be around 4% growth for a single billion which is quite easy and done multiple times historically.Low millions sounds about right, even if the starting colony size is 10,000 in 300 years you've got around 70 million. This is VERY low for an entire world, and you're only going to be able to exploit a fraction of the planet nevermind a solar system. At least it's low compared to the billions of capital worlds.
And guess what? A 7 million population actually requires far more shipping than I'd been assuming for low population planets. So I'll have to revise dropship numbers even higher than I had been.
Plug in a more reasonable 5% growth and you have 22,739,961,286. Yes, 22 billion. The 3% LordSunhawk plugged in is what you'd expect from a fully developed world with no free land to be colonized, ie. the modern day, not anywhere with a frontier. Historically when there's land, people tend to have large families in order to obtain said land. I would hardly expect an actual 22 billion from such a world but hitting a single-digit billions in 300 years is dead easy, that'd be around 4% growth for a single billion which is quite easy and done multiple times historically.
This is exactly my point, yes. Start from a effective population of 10,000 (which is pretty much the *minimum speciation*) with a realistic growth rate, and you get population in the high millions to low billons very, very fast.