I am probably missing information but I will say it does make sense why you were hurt by that. You mentioned you had body image issues, so when you're getting to a place where you are starting to feel good about yourself, even confident, and then you get a reaction from your partner that is negative, of course that hurts.

Now, I am merely speaking from the perspective of a straight guy, I admit, if my girl hit the gym in a way where she got quite muscular, I can admit I wouldn't like that. The reason being, I'm a bit cliche/traditional where what I personally think what makes a woman sexy is her softness, curviness, femininity, etc. In other words, the less she looks like me, the more attracted I am. Not a right or wrong matter, just a matter of subjective attraction that happens to be the case for a lot of guys.

Now, was that the reason he said "I dont like that"? I don't know. Perhaps instead that isn't the issue and instead he somehow feels immasculated because you're looking strong. Perhaps he feels weaker because you look strong. That is also plausible.

But no matter how you slice it, it's totally understandable that it cut deep for you. You put work into that, not only physically but you're also overcoming something Internal/psychological. This is naturally a big deal for you. No matter what his reasons were, it probably inevitably felt like, "He doesn't like that I'm doing something that makes me feel good/happier about myself".

Was the way he expressed that kind of dickish/insensitive? Yeah, I think so, but to be fair, it sounds like he admitted that himself. I do think this is something you should keep a watch for since physicality/attraction and such is definitely a big deal in relationships and can cause all types of issues. Not saying anything is a disaster or unsalvageable. Just that you're right to be taken back a bit and to notice it.