Calico, honey, those people you see claiming to fit, that's exactly what they're doing, claiming. They're lying their asses off, rather they know it, or not. I have seen people who were older, and could fit in baby diapers. One is someone I knew from after school care, from 4th, through 6th grade. She was around my age, I think, a bit older. Sweetie, there's a reason she could fit. She was severely athetoid cerebrally palsied. She looked sick, very sick. You could see her every bone. She was fed, but needed baby food, because she had so much trouble cancelling her body's unwanted movements, that consciously chewing unaltered solid foods, wasn't possible. Her body always moved, everything, eyes, mouth, arms, legs, everything. Her body incinerated calories, like a furnace. She couldn't maintain weight.
Another friend had the same type of CP as she, but was a bit better off. His parents fed him solids, but, modified them. For example, I remember feeding him a peanut butter and jelly sandwich, soaked in chocolate milk, once. We were kind of crushing on each other. That's why I got to feed him. Letting someone feed you, when you can't do it yourself, is a big trusting deal. As I type this, I feel honored he asked me to. He could've probably fit in baby diapers, but, again, he couldn't maintain weight, in spite of being able to eat more than most full grown men. His everything was always moving, only a bit more controlled than my other friend, I mentioned above. Like her, he could not walk, talk, feed himself, or sit without support, but, he was able to carry spare undies in his backpack. We taped a few written phrases to the wall, numbered them, and he'd look at the list, when he wanted to say something. If what he wanted to say wasn't there, we'd go through the alphabet, and he'd wrinkle his forehead, and look up, when I got to a letter, until he'd finished his thought
There was a boy older than I, with Down syndrome. He looked about 6. He wore pull-ups, real deal, Huggies Pull-ups. He was so tiny, because of the hole in his heart.
There are grown people that can fit, but, anything smaller than a goodnites or underjams L/XL, and, I guarantee you, there's something very wrong.
On another note, it's kind of hard to defend that we aren't pedophiles, when some of us are actively buying diapers intended to fit children. Not saying it's a perfect way to look at things, because, by that logic, anyone who's ever bought a chime rattle for himself, or herself. . . (Adult chime rattles don't exist yet. Do they?) Just saying that's why adult diapers, with baby prints, are a thing.