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 "Not Enough People For This World."
You get this impression not only in Harry Potter, but in Game of Thrones and other fandoms. We see, or at least hear of, armies and other groups made up of thousands or millions of people, and we know we're dealing with a world fairly vast and large, yet all the widely-strewn characters keep bumping into each other within it. And when you do need an army of millions, they aren't there and you end up with seven or eight familiar faces doing the heroic defending. (GoT does have people hiring entire armies, I know. But at the same time, they also frequently have people traveling hundreds of miles and randomly encountering someone they know. And you occasionally get the weird impression that some entire kingdoms have, like, fifty or sixty people living there.)
TV Tropes does have the "It's a Small World After All" trope and the "Contrived Coincidence" trope, which both overlap what I'm describing, but are not quite the same thing. Thoughts? Anyone else have the Not Enough People For This World impression in other material?