I think comfort reasonably covers all of that - weight or SH related, from PP it sounds like even if they are hotter dressed than way, they are more comfortable overall without showing parts of their body they are uncomfortable with. Skin cancer, you’re more comfortable covering up than risking health implications. See also religious and modesty reasons.
In fact, if I had something to hide that meant I had to wear unseasonable clothes, I’d rather someone ask so I can say “now you mention it I am warm, can we go somewhere air conditioned”, given it sounds as though the person asking has concern for my wellbeing.
I was meeting a friend somewhere the other day and she mentioned she was driving. She’s never said but she rarely drives and I suspect it’s due to anxiety. So I asked if she was alright driving there, she said she’d found a longer way with no motorways so she’d be okay, I offered to pick her up, she was delighted and took me up on the offer. I suppose I should’ve been more sensitive to her anxieties and insecurities and not mentioned it…
There are about a million things people say that can be misinterpreted and taken offensively. People can’t avoid saying these things or we all become too scared to say anything. In my view, resilience has to be the only way forward to keep your sanity.
I wouldn’t be suggesting resilience is the route if the other person had said “you fat cow, what are you wearing?!”… I’d be suggesting boundaries and ending the relationship. (And I’m not for ONE SECOND suggesting the OP or anyone else is a fat cow, I’m using a common, rude and hurtful fatphobic term to make a point).