In the original books, Harry's cohort was born ten years into an extremely bloody civil war. I always assumed birth rates were extremely low for Harry's age group, which would imply that the overall population is much larger than what you'd extrapolate from class sizes.

In that case, shouldn't we see evidence of a baby boom occurring immediately following the end of the war, probably in the form of the years after Harry's being noticeably bigger than those that came before? canon!Harry is rather unobservant, but you'd think he'd have noticed at least that.