Why my boyfriend thinks he's funny/clever?

if your weight is something that you take very seriously then so should he. it all depends on how you handle the situation. have you made it something that is ok to joke about? are you actually skinny enough to be considered anorexic? if you haven't specified or established clearly what is ok with this thing which is obviously a part of your relationship dynamic, then he had no right to preemptively make light of the situation. if you feel that this was just a misguided way to make you laugh and/or you can forgive him for something that he didn't make, only saw and sent, then take it like he probably meant it. just a laugh and let it go. if this is something that you're sensitive about then you just need to talk to him about it. if he's a decent guy he'll understand