It makes no sens FOR YOUIt makes no sense that every planet has variable district slots but only 12 building slots. Make both of them scale with planet size for a little bit more realism.
But the thruth is that buildings are tied to city districts, and those are tied to planet size so... it is what you are asking for - bigger planets has more building slots than smaller ones, unless your big planet is super huge farm, and small planet is one planet-size city... which of those two should have more "buildings"?