I've been thinking about that, the population density thing, and I have an idea. What if there was a 'max' population per km^2 of surface area of a body, and to populate beyond that required a (very small amount) of infrastructure? For large planets like Earth it would be a fairly high number, say 10 billion, but for small moons it could be much lower - say 100-200 million. Luna, for example, has about 7.5% of Earth's surface area (So ~750 million) , while Mars has 28% (~2.8 billion). So we're talking really huge populations by normal game standards. And the required infrastructure should be small enough that the natural (via trade goods) production of a world of that size should be enough to keep up with it - you just won't be exporting infrastructure from those planets to colonies any more, as they need it to support their own growth beyond what the planet can normally handle.
The problem is that I think it isn't possible in the current infrastructure requirement system to have a certain amount of population 'excluded' from colony cost requirements. You want the population up to the 'limit' to have a colony cost of 0 and from there onwards you want about an 0.1 or so to represent 'colonising' previously uninhabitable areas of the world, or raising habitation density, or whatever.
