The more people your colony has, the smaller the "colony size bonus" becomes, eventually going into negatives (before reaching the cap of -4).
And, as you may know, people don't want to join some kind of a dump - you can only accept half of the newcomers if colony morale is below 2, and cannot accept anyone if colony morale is below 0.
But it's not just that - the subsidy depends on colony morale.
Assuming that more or less everyone is employed, morale of >3 yields you 10/person/turn, morale of >2 yields 6/person/turn, morale of >1 yields 3/person/turn, and morale below 0 will lose you money, which is a kind of situation you want to avoid at all times.
While consistently maintaining a morale of >3 mid-game can be tricky without prioritizing community-related research options (i.e. getting a park or a restaurant before an electronics factory), you can always strive for >2, which will provide a steady stream of income (600 at population of 100) to use for buying resources and/or terraforms.