Hi,
I'm new here but for what it's worth. I think you should be really honest with her in a non-agressive way.
By that I mean, tell her that when she talks about how she feels about overweight people, you feel like she is talking about you and that it therefore hurts your feelings.
Tell her you think it's possible that she is currently just ignorant of your feelings and that you hope by bringing it to her attention, she will cease and desist. However, if she doesn't, tell her that you will have no choice but to take it further.
I'd also make a note of the date you had this discussion as well as what she says in response. If she doesn't stop, I would start making some notes about what she says and the dates she says them for future reference if it ends up that you do take it further.
Hope that helps. It sounds to me like she's just ignorant of how her comments are impacting on you. Most "normal" people respond positively when its brought to their attention.
Christie