Another possible reply, "Because it really wouldn't be fair to people like you if I was this awesome AND skinny."

Wow so rude

Or when she asks why your taking her food, you could say it's because your working so hard

I think it's HER life that sucks. If she truly believes that thinner = better, then she'll never be able to relax in her own body and will spend her life alienating less superficial, cooler people like you.

Coworkers should be minding their own business. They shouldn't talk about other people's personal lifestyle. If she wants to be a super skinny scumbag then let her be.

Na be like no but you should tho and if not then be like what I do is my own business and I might report you to human resources

This could have been a good thing, at least initially. perhaps the coworker thought the OP looked like they lost weight, figured if she lost that much weight so quickly, she must have been on a diet. On the other hand, OP could have made the best of it and replied "because I'm beautiful the way I am, thanks".

Just say sorry I'm too busy shoving this triple cheeseburger up your ass

try not to take it personally every can use a diet

shots fired!