I would say it’s a combination of station and planet but possibly also economy. Not totally sure on economy side but one refinery system of mine has around 4 million pop at this point with a Coriolis and T1 planetary (regular non-atmospheric HMC with geologicals) while another on an also non-terraformable world with organics, giving it agriculture (and terraforming) on top of extraction and refinery has a population of 10+ million. Someone mentioned agriculture might have higher populations anyway.
Saying that the second refinery was also built post-update 3, which may play into things.
Meanwhile my pre-U3 Orbis primary around a (terraformable) water world is either approaching or just about crossed 800 million with relatively little else in the system yet. One space farm, and some installations(relay station + either 2 satellites or 1 satellite and 1 comms). I’ve yet to put anything on the terraformable high-metal contents to ascertain their value, but also not likely to happen until we have more info on the future of economies in this ‘beta’.
I have a terraformable HMC with biologicals, T1 commercial outpost, T1 civilian planetary port and 2 refineries. Pop 4.1 million. Gained about 200k last pop tick.