"High pop" and "low pop" worlds could be determined by focus or Buildings
for example, various "max population" bonuses could only apply to world with a Particular Focus (ie Subterranean Construction... +3 max pop to Industry and Reseach focused worlds, +1 to Mining Output on Mining focused worlds)
Megalopolis building... +100 max pop: Food Production=0 (requires 10 Food per turn.. to deal with the extra environmental costs)... You would be limited in how many of these buildings you could build, snce you need the food worlds to support it. (and Industry worlds need Minerals to be useful anyways)
The problem is that the game is not designed for control of individual planets, so choosing where to have high population may not be an option. Probably it is best that population just grows as it grows, like in real life - governments try to control these things but they tend to fail.
But, I do think that the focus should affect maximum population. Trade worlds should naturally grow, farming worlds should use all available space for farms and not be great places for teenagers and young people, who would naturally try to move away. Good recruiting places for the army ... (I am not suggesting migration of population units, just that population growth could be assumed to include migration)
Gaia worlds should have the 'right' amount of population not the most. Planets covered in cities should have the highest population. Maybe Imperial Palace should increase maximum population, since it makes sence that the capital will have lots of people living on it.
Maybe all buildings should increase max population, since these are the focal points of the Empire, where people would tend to concentrate. Planets without buildings are the backwaters, just there producing resources.
In my current game of FreeOrion I found a planet of natives which do not need food. This seems to also give them the side effect of never growing in population and not being starved by blockades
Which one? That was not intentional.
They are called Setinon, they are on an ocean world with very slow rotation. Their icon looked like a hairy bacterium.
20 of them are making 20 food - 10 for slow rotation +20 for self rotating crops + 2 for Orbital farms, for a total of 32.
Their food consumption is 0.
When I got Subspecialised Treatment their health went to 25 and they are now slowly growing in population.
Also their construction is just -20 due to very slow rotation. No +20 for their species.
I cannot attach the .sav file to this post.