I'm glad you agree... and sorry for some of your crappy experiences.
There are people who do lead unhealthy lifestyles and if you care about them, then yeah, it might be an okay idea to CONSIDER bringing it up with them POLITELY... but if it's a random person or someone you're not close to? Then STFU. Chances are they don't want or need your opinion or "advice," anyway. Especially when it's just, "Yeah, skinnier/fatter/etc is hotter." Because that helps who exactly?
I think it's fine to have a preference for a certain body shape or size or whatever. To find one more attractive to you than another. You can't really control that stuff... but having a preference doesn't mean anyone who doesn't fit that requirement is unattractive or in need of your criticism.