I realized something

[Night Wolf]

Do not question it!

[Genevieve]

Weren't Draco's buddies fatties? Also, Ron ended up chubby in the epilogue.

But yes, if anything, Hogwarts students should have a higher incidence of obesity than the wider British population. The magical lifestyle seems very sedentary, just levitating stuff to yourself and whatever. But there was a flip load of stairs.

HOWEVER, if you did just take the core 5 or so characters from any book/movie/tv show ever and extrapolate them to the population they are representing, you would assume there are no fat people anywhere ever.