Ok to be fair, if you brought me to HR over a comment like that without talking to me about it first I would make your life miserable. Lol, when I was done you would actually be harassed. Don't be a tattletale before you act like an adult. Go up to her and say, "look, your comments are getting on my nerves. This is the one time I'm going to ask you- don't comment on my weight, or anything related to it. This is a courtesy, next time I go to HR." That's still a little harsh, but once you've given fair warning if she keeps it up, then the consequences are her problem, not yours.
This person is stupid, but I don't think she has any idea that what she's doing could get her in trouble and I doubt she is doing it to be annoying and hurtful. Sometimes people just make comments to have something to say. On the slim chance she is intentionally being a jerk, the trick with bullies is to be the bigger one. Give her fair warning that you're tired of the comments, preferably open with something like "your comments are really bothering me" and then progress to "stop your crap or I'm going to get you fired" if you need to. Sometimes people don't realize that they are being stupid and hurting them professionally can accomplish much less than if you just confront them first.
