Why do people think it's ok to tell someone thin that they should eat a sandwich, but never say to a fat person they shouldn't eat that?

Both are rude but it's much worse to be fat so it is considered more rude.

I don't think anyone ought to eat a sandwich, thin or otherwise. Grain was never good for us. Bread is junk food. Totally

Its not ok to do either

I dont care what you think about my weight. That you think that i'm those girls who don't eat or i'm always up to no good and don't eat. This is my natural weight and i am healthy.

False premise.

Commenting on other peoples food choices is rude and serves no purpose. It is just a passive aggressive way to harp on body image.

It seems that most people do not consider it offensive to say something about eating to a slender person.

I don't say it, but think to myself - "eat a cheeseburger" when I see an obviously anorexic girl. There is such as thing as being too skinny.