I think it is 'socially acceptable' to say 'skinny' but not 'fat', because people aspire to be skinny, no-one really aspires to be fat.

Now just because that is what society 'accepts' doesn't mean it isn't rude to say either. It is rude to comment on anothers weight regardless of if they are fat or skinny. On of the factors that lead me to gain weight was being teased as a child for being too skinny. Looking back at photos, I wasn't too skinny, just had a slight build. Now I don't have to worry about that, instead I get to worry about people commenting about how much 'bigger' I am and how I look good loosing weight, but how it is a 'shame about my belly' (lots of loose skin makes a buldge). People are idiots. As if I cann't see and judge my own flaws without their help.