He keeps telling me to get fat

First off, I’m 17, 5’3 & weight about 112 lb which is perfectly fine for my height (or so my doctor tells me). I’m healthy & I tend to eat a lot but I don’t gain a lot of weight because I have a fast metabolism. My bf will often tell me “you should get fat” & if I ask why, he’ll say “so I can look skinny next to you” but I know that’s not why, he’s actually losing weight & he’s neither fat, nor skinny whereas I’m petite. It’s starting to annoy me because I’ve always been self conscious of my body & having to look for t-shirts that are xs because not even small will fit me. I was skinny shamed throughout my entire life & im not anorexic, never was, I simply can’t put on weight. I don’t want to say “stop telling me that” or anything because I don’t want to come off as butthurt (which I am).

He’ll say it so random, out of the blue.

Included a picture of my body because I don’t think I’m THAT skinny. Idk what to do /: I’m getting frustrated at him.

This is us in comparison. As you can see, no one will call him a blue whale just because he’s standing next to me.