It was very rude to comment on your shape, and even more so in your son’s wedding photos!
Some people can’t help it, it seems - they think they have a right to comment on personal matters, when they really, really don’t.
Ignore her if you can, and if she says anything further you could suggest that you try to lose some weight if she tries to learn some manners.
If it’s any consolation, it’s my son’s wedding in a couple of weeks, and I would love to be a size 14 on the photos . I’m quite sure your son will love you just as you are, and in any case will only have had eyes for his bride. I hope you had a lovely day, and that your MIL’s thoughtless comments haven’t put a dampener on it.