olivia I read that as shovetheholly writing from her gran's perspective, not that she necessarily feels that way herself. I appreciate I may have misread it though.
There are a couple of ways you could tackle this OP.
A) Next time someone comments, say "it might have been acceptable to make comments about someone's size back in the day, but it isn't now so I'd appreciate it if you'd stop. Thanks" and then change the subject.
B) Ignore and rise above it, while feeling sorry for them that they are so superficial to be so concerned about appearance.
C) Every time someone comments, say "what a coincidence, I was just thinking you could do with some meat on your bones, you're looking a bit scrawny".
A is the mature option, B is okay if you really think you can do it, C is the immature and childish approach so you might want to think it rather than say it but it depends on the culture of your workplace (and how peed off you are!)