Why is it socially acceptable to ask someone, Why r u so skinny? But not so to ask, Why r u so Fat?

It's not socially acceptable either way. I was a twig growing up and everyone make those comments to me. It ruined me. I tried so hard to be their 'ideal' normal girl and at one point, I even ate spoonful of butters just to gain some weight. Now, I give them all the finger. It's none of their business.