I want you to know that he is a pervy idiot. You do not have a big bum. It’s simply not possible ant your weight. Even if you did, or ever do in the future, it doesn’t matter.
I say this as a much older woman who was terribly thin in my 20s.
On one single occasion, I remember how I ate a pasty out and about (350 cals, I always thought checked this stuff!). But it was a big looking pasty. Anyway, a builder whistled at me and shouted, “watch out you don’t eat it all or you may get fat”.
He probs thought he was being cute. But as with your example it ruined my day, lots of fretting, and made me stick to an even more restricted diet, which was painful and hideous for me.
honestly, honestly, it’s a pity I gave my ED or indeed that man any time. I wish I’d focussed my energies elsewhere. I don’t have answers since a little ED monster is still within me, 30 years later, but now I don’t care (plus no one perves over me at my age!). Take care.