Ever wondered how many students are at Hogwarts? J.K. Rowling's response in an interview back in 2000 gave the answer: about 1,000.
But when you look closer, that number doesn't make a lot of sense. Why are there so few students in Harry's class?
That's almost half as many students as there should be, if Hogwarts has 1,000 students as Rowling said.
TL;DR: There are fewer kids in Harry's year and thereabouts because of the war with Voldemort.
Which is a pretty sad thought.
If that was the case, we should expect a bit of a baby boom in the years under Harry, as that tends to happen following major wars.
Of course, none of this explains the sheer numbers of the Weasley family.