You’ve probably heard things like
“Oh my god that dress makes you look so skinny!”
“Oh my god you look so skinny in that picture!”
“Wow I look so skinny in this!”
“Ugh I look so fat today.”
“Ugh this shirt makes me look so fat.”
“Ew don’t post that picture I look fat in it.”
Thin privilege is skinny having a positive connotation in every situation.