While all bodies have a hard limit on space available, you can work around it somewhat. By altering (requires SM mode) the population density modifier you can get larger/smaller populations on any given world. I tend to reduce it on my current game due to my Empire being more of an open green spaces sort of culture, opposed to massive high rise cities. This has the side effect of forcing me to colonise other worlds where before I would ignore them due to lack of minerals etc.