@Xeredge if this freaks you out, I recommend avoiding Mitsudomoe. Doesn’t lewd them, but there’s plenty in that that made me uncomfortable. I feel a lot of comfort in this kind of thing is how the characters are portrayed. I feel the tone of this manga seems closer to that of a comedy high school manga, so it hasn’t really been a center point in my mind that this is supposedly middle school.
I admit I'm binge-reading this and watched the anime, and had kept in mind they were middle schoolers the whole way through from the beginning, but I do keep being surprised at how easily people lose track of that fact. I mean, the whole rest of the class look like middle schoolers, don't they? It's just Yamada that looks mature, and it's because of genetic lottery. And her parentage, which gets covered later.
She has consistently been depicted as someone that stands out in classes. Pretty sure there was a piece of splash art that was meant to be from her elementary school days, where she appeared to be maybe twice the height of her classmates. In fact, just found it. This one:
Vol1, Ch6.1, Pg1. You can see they've all got shirts that say '5-1', meaning that they're all fifth graders, and Anna is towering over the whole lot of them because she has tall genes. She is just genetically predisposed to be bigger than her peers, in just about every way, and it makes her appear that much more mature. It probably gives her insane metabolism that can both explain her gluttony and her ability to stay slender despite said gluttony.
I'm also uncommonly tall, and have been since my teenage years, and I've had a voracious appetite and can never seem to put on any excess body fat. I'll also note that I was diagnosed early with ADHD and took ritalin through elementary and middle school (but then switched to Adderal in high school), and it's said that stunts your growth, so I wonder if I would have been even more massive if I switched ADHD medication earlier. I'm 6'7" (around 2 meters), for reference, and only weight around 185-190 lbs (~84-86 kg).
But, anyway, yes, the author... she seems to not shy away from certain topics or depictions as much as some other mangaka. She's pretty bold, arguably, like Anna herself. There might be some self-insert to an extent, there. Either Anna or Moeko. Much like the creator of Angel Next Door, Saeki-san, has been speculated (in comments on the manga adaptation here) to do self-insert with Mahiru and imagined herself having such a thoughtful and caring husband-not-husband like Amane, if she's not married herself.