So habitats really just improve upon the agri/social/labor ratio, and thus have lesser effect if that ratio is already naturally good in some place, like zero or even already <2 colonies.
They don't really "improve" said ratio. They are pretty much earlier, more expensive, but also more versatile equivalent of underground infrastructure. Their main purpose is to allow the player to colonize planets which either have wrong gravity or such large colony cost to make industry impossible.
To be more exact, the percentage of people working in social services is the same in orbital habitats as is everywhere else (the percentage growths with population, capping at seventy five percent) while agriculture and life support are entirely omitted. In theory that means much more workers on planets with colony cost above 2.0 (as you need 15% of your population in agriculture and it increases with colony cost) but they are so expensive that they are rarely used and when planetary population is low (less than 25 million) you have such a large percentage of people in manufacturing (only 40% is working in services) that there is little difference between habitats or infrastructure.
