This is such an interesting topic.
I'm currently reading a book on my kindle called something like How to eat like a slim person. It's actually not very much about dieting but a lot more about being mindful, thinking about what you're eating, eating slowly and not using food to medicate for other issues.
I've been consciously trying to do it over the last couple of days and am astounded at how little my body actually needs to keep going - I think it will be harder when I am back to work next week though as it's constant go with school collections as well and rushing everywhere - thinking about it I always feel hungry at work, but I suspect it might be because I'm stressed and anxious.
FWIW I'm a generous size 14 and I don't sit eating haribos and cake all day, in fact I'm not much of a fan of any sweet stuff apart from chocolate which is my nemesis. My best friend has a very petite build ( I'm taller with broad shoulders) and whilst she does not diet at all, she hates feeling overly full and always stops once she's had enough. My cousin is the same, I was blinking starving when I stayed at theirs, she sees food as fuel only really, so if I'd known there would be nothing between meals I would have eaten more then.
What I'm saying is that naturally thin people seem not to think about food very much at all except when they're hungry.
Screenshotting - we get the opposite with DS age 10. He's stocky and broad shouldered but not fat and fits into age appropriate clothing. People always seem to send stuff a few years older than him, still at least he grows into it.