Harry Potter author J.K. Rowling has previously gone on the record about how many students are supposed to be enrolled in Hogwarts, which sat at 1,000 students.
After a closer inspection of the Harry Potter films, however, there seems to be a huge gap between that 1,000 student number and the actual students in Harry's various classrooms. After Buzzfeed did some counting, there are 18 students in the Gryffindor's original class, which is well below what the class size should be according to some simple math.
If you take 1,000 total students and divide by 7 (years), you'll get 143. Take that number and divide it by 4 (houses), and you'll get 35 students per class. Harry's class of 18 is well below that bar.
According to Tumblr user Marauders4evr, there might be a very specific reason why Harry's class is so small.
"Oh my god…I just realized something.
For years, we've all wondered how there can be 1000 students (according to J.K. Rowling) in Hogwarts when there are only a handful of students in Harry's year.
The math doesn't add up. We've all just assumed that it was an error. But what if there's normally dozens of students in each house, in each year?
What if Harry's year was the exception?
What if there were less students in the Hogwarts Class of 1998 because the period when the other kids would have been conceived (1979-1981) was when Voldemort's reign of power was at its peak? Between the dozens of adults who joined the Order, the dozens of civilians who were killed in Death Eater raids, and the dozens of adults that didn't want to bring a child into the world, just then…It's actually entirely possible that there was a baby drought for a few years in the wizarding world, leading to a smaller class size a decade later."
It's a solid theory, and it makes sense that there would be a baby making drought during Voldemort's reign. While it's sad to think about, it does bode well for the Voldemort-less future.