As a former skinny girl I can confirm.
Comments like these made me remember when I was a teen and I used to get tons of comments about my body, especially from my friends (who were not fat, just normal with a bit belly). If you're skinny, people tell you to eat; if you're fat, people tell you to eat less. As long as one isn't sick, no one should comment on anyone's bodies (unless you're asked or qualified). You can't even post a damn picture of yourself without anyone trying to speculate your intentions.