The "Not Enough People For This World" trope

I see that the small class size of Hogwarts is a trending topic lately. The "Voldemort-era anti-baby-boom" explanation as shown there on Buzzfeed may actually make some sense. But on the whole I'd attribute the small-ish cast to a trope you see a lot in fantasy, sci-fi, and other works involving extensive world-building. As far as I can tell, TV Tropes doesn't have a name for it (or maybe they do, but I'm not searching deep enough), but I'd call it something like &qu...