I've been reading y'all's comments on my question about fat wizards yesterday, and y'all have awesome points. But, now that I'm less almost-asleep, I wanted to expand a bit upon more what I was talking about.
I know in the movies and in RP, wizards are generally by default "pretty" simply because we have Hollywood to thank for that: actors and actresses were cast in the movies, and they're pretty people, and the people we have to choose from for PBs for RP are actors, actresses, musicians, models, etc, and they are generally rather pretty, too.
I was focusing more on in the books. I haven't read them in a while (that might be my next series reread), so I might just not be remembering properly, but ... well.
Harry was raised a muggle. He might be a wizard, but he comes into the wizarding world the same way the rest of us do: with an outsider, muggle perspective on everything. It's a great plot device to introduce the reader to a new world and rules without having to outright go with Department Of Backstory, or "Oh, Ron, I know exactly how this works, and so do you, but let's talk about it anyway just ... because." Which means, regardless of what wizards raised in the wizarding world might think of beauty, our main narrator is an outsider like us, so he's bringing with him the same muggle prejudices we do.
And, yes, we don't get a lot of physical description of a lot of characters, and a lot of them just have a few flyaway comments about them: Hannah Abbott's blonde braids, or Hestia Jones' rosy cheeks and black hair, or Emmeline Vance's ~stateliness.
But he does very specifically seem to note people who, to his "muggle" perspective, are particularly skinny (Trelawney) or particularly large (Dudley & Vernon, Crabbe & Goyle, Millicent, Fudge, Sluggy, and everyone else that y'all mentioned in comments). I remember reading somewhere (an article or book on description and reader prejudice) that, with lack of description, the reader tends to "fill in the blanks" with a general sense of "normality": they either picture all the non-described people as looking more or less like themselves, or they picture the non-described people as more-or-less resembling the main, narrative character (in the case of HP: white and relatively "normal" in terms of weight).
It feels sort of like JKR is doing this with the vast majority of the wizarding world: by introducing us to the wizarding world by way of a muggle-raised character who doesn't know anything about this world and has similar body image prejudices that we have (though, admittedly, Harry's are British and a little less Hollywood-directed than ours here in the States), and then specifically identifying some characters as overweight and some characters as underweight, she is sort of indicating that everyone else in the wizarding world is a more normal, healthy weight, because our muggle-prejudiced guide into this world isn't noting "wow, all these people are particularly heavier than the majority of the muggles I've grown up around!"
idek if I'm explaining my thoughts right, but ... idk. Though I do like the idea of the magic itself sort of burning calories (so does that mean that since Trelawney's a stick, she's super-duper magical?), or the wizards basically being able to use magic to make themselves look (or even actually be; magical weightloss?) skinnier.