Qwertier

Guys i found the explanation!

Wait. Wait. The entire fandom has been arguing for years over whether to take the 40 students in Harry's year and multiply by seven to get Hogwarts' total student population (280)or trust JKR's estimate of the total (1000) and assume there are more students then 40 in Harry's year, even though we have a list of all of them.
But. You guys. Harry was born during a war against Voldemort, who had no qualms about killing infants and who certainly had no problem killing the young people Harry's parents' age.
Harry's year is unusually small, and maybe Ginny's is too. Things picked up again after the war, when there wasn't the threat of death hanging over everyone's heads and taking young soldiers away from their spouses or significant others. And it didn't really start until the Marauders were adults or almost adults, so it shouldn't have been an issue shortly beforehand, either.
Harry's year is 40 students, instead of the expected hundred-odd because of Voldemort."The" girls' dorm and "the" boys' dorm are he only two dorms used by Harry's year, but of course there are closed-off rooms, maybe ones that have disappeared because they don't exist unless they're needed. You know those empty, unused classrooms that I seem to recall hearing mention of once or twice? They're for splitting classes that would be too big. But that's not needed got Harry's year (it might be needed for the year below Ginny's though).
Imagine being a teacher or one of the older students. You've been to sortings before- one with a hundred kids or two hundred kids, that's what you're used to.
Then you sit down and watch the children file in for sorting. And there are forty of them. And you count back in your head and you realise these are the children conceived during the war. And this year is small. And so is the next.
But then the post-war baby boom starts using all the closed-up dorms and classrooms, and Hogwarts is back to normal.