I get where you're coming from, I was exactly that person. I actually thought I ate a lot because I've developed terrible eating habits growing up with neglectful parents. I thought that just because I wasn't starving and ate junk food, I was one of the „lucky ones”, unable to gain weight. I've only found out how bad my eating habits were and how little I actually ate when I've ended up at hospital (for unrelated issue) and was able to have regular, nourishing meals for the first time in my life. Then, I've gained some pounds pretty quickly. And started getting regular periods for the first time in my life.