If you think that she is being mean on purpose than call her out on it at the time. If you leave it that’s when you start to overthink things, you should have just said “that’s mean” when she said it and see how she responds. And sometimes we don’t like to hear the truth and the older generation just tell it as they see it, maybe she just means you would look better in that particular dress if you shed a couple of kilos

How are things now?

How are things now ?

Relationships with MIL and relatives can change over time; sometimes good and sometimes not so good! Hope things are ok for you and your MIL.

Hope all has worked out for you and your MIL.

Has your relationship improved with your MIL?

This might help. I had a rocky start with my MIL. Finally I sat down with her and basically said, Right – woman to woman, not MIL to DIL – what is the problem?
Turned out (thanks to nasty SIL) she thought I was gold digging. Once I pointed out there was no gold to dig, our relationship improved vastly. She died 5 years ago, and I still miss her badly.

Perhaps you are overthinking things or you are seeing things plain & clear. I think you & your fiancé are a strong unit & this is all that matters. Tell her to butt out.