The Pops do grow into place with food-consumption determining how quickly they become an active Pop, though, IIRC? It would be interesting in a DLC if smaller space habitats or nomadic Life Ships could each contain individual fractional Pops that limit the growth and have lower output. So locked at +0.1 or what-have-you per expansion-upgrade. Perhaps even multiple fractional Pops at once capped off, for political dynamics.
The flipside to the benefits of constructing them would be that A) Expensive start-up cost (less for stationary of equivalent size, as you bulk up Life Ships more for extra capabilities), B)
SIGNIFICANTLY more vulnerable during wars. Stay vigilant, control space-superiority. Think the destroyed habitat Heliopolis of the Orb Union or the Junius Seven tragedy during the early episodes of Gundam SEED or the later attacks on their home PLANTs threatening their entire homeworld-equivalent.
Would also help advanced-but-small tall regions, unable to expand, not simply be out-massed by larger nations? Some large-scale habitats and powerful military stations in a defensible position.
You'd need to get into orbital megastructures - rings around earth for example, to get enough area to plausibly house the many hundreds of million, if not billion, people representing a single pop point.
You are heavily underestimating just how massive the potential population able to be housed by such a colossal installation is. Like, that is a continuous human-constructed urban habitat stretching across multiple continents. Food production aside, it's... quite something. Remember the magnitudes that they had to scale down the available space in the Ring World to not have it be "
Basically if you control this your opponent would have to colonize the rest of the galaxy to compete".
There's a reason a civilization on the level of the Culture abandoned terraforming in favour of more optimizable Orbitals.