Edited to add some numbers on Hogwarts and Wizarding Britain numbers from Jo: I found this interview from JKR regarding Hogwarts numbers:
Then I’ve been asked a few times how many people and because numbers are not my strong point, one part of my brain knew 40 [original characters she had created for Harry's year], and another part of my brain said, “Oh, about 600 sounds right.”
600 in Hogwarts doesn't seem so small after all. But that paints Harry's year as being more than half the usual yearly intake ((600-40)/6) of around 90, which then goes on to give up a total population size of 13,500, take around a couple of thousand for the casualties of the two Wizarding Wars in the last century, and that's still a fairly decent number.
But then that same interview goes on to say (in the very next line):
Let’s say three thousand [wizards/witches in Britain], actually...
What?! That means a Hogwarts year should have no more than 20 students at most! Harry's year is overpopulated; and total Hogwarts population should be something like a 150; and casualties under Grindelwald and Voldemort were massive percentage chunk of population. Or maybe most Wizards only live up to a 70-ish, same as Muggles, except a lucky few. That's the only way to explain those numbers.
Thankfully, JKR gives us an out and ask that we: "don’t hold me to these figures, because that’s not how I think."
In Conclusion: A 'corrected' average Hogwarts size of around 400-450 might be a good compromise between JKR and McNumbers. Harry's year in pretty small with 40 students, but the average yearly intake is 60-65 (two dorms per year per house per gender with 3-4 kids each (explains MWPP), instead of one with 5); leading to a total British Wizarding population of around 10000. The rest of the points in my answer still apply, I think.
And the explanation for this, as given by JKR herself:
Oh, Emerson, my maths is so bad.