I think she's likely to be pretty sensitive about her weight already, and if it were me, I'd resent my Mum saying anything, however tactfully it was said.
It might not seem reasonable to you, but I think by dropping hints, she'll feel "got at" and annoyed by your comments, and it won't help to sort the problem at all. In fact it will make things worse for her. If you've never had the problem, it's so difficult to understand - people think, well just eat less, it's easy!
It really is all in the mind, and hopefully she'll do it when she's ready. If you could possibly arrange a group of family and friends who all go to a fun dance class each week, such as ceroc, as a social thing together, that might help her.