It's really disgusting how much media can shape your ideas of how a body should look.
I remember seeing Lindsay Lohan in Mean Girls and thought she looked very nice, "normal". But somehow media was already onto her and started calling her a bit chubby and that she had a bigger bust. It must have been slightly after that that poor Lindsay got thinner every time she was in a picture. At her very thinnest I found her too thin, but for a long while I wished I looked like that. While I was always a low to normal weight, I never had a flat stomach and always a bit of chunk at my hips, so the early 00's with their effing low rise jeans really did a number on me. And don't get me started on those weird diagonally-cut tops that would expose half of your stomach. I was semi-professionally swimming back then and saw slimmer girls with flat stomachs all the time and thought they had it so easy, they didn't need to suck it in. I thankfully never underate, I was always very aware of the threat, but it definitely is something that was lurking around in the back of my mind.
My boyfriend kind of once said something that I think eluded to him liking slim girls (but with boobs and butt, of course) because that was the ideal when he was a teen and started to discover girls and sexuality. He probably has a point, he was primed for this, just like I was for the want to look like that.