I'm not sure if you want me to answer there or here, but I'll answer here in case you were looking for a different kind of response.
That comment is pretty awful. 
Do you have a sense of why she does this sort of thing? I mean, does she (misguidedly!) think she is somehow being supportive in some odd way? Or does she just speak without thinking? Has she had an influence on your attitude to your body/ relationship with food in the past, do you reckon? Or does she have her own issues maybe?
Don't feel like you have to answer any of that if you don't want to, just wondering if any of them ring a bell for you.
I think it's definitely a feminist issue, how you relate to your body and how your mum influences you. It's awful that she is focussing on how you look, not how healthy you are. Can you tell her how upsetting that is? Because obviously though most of us do diet because we care how we look, as your mum your health should be the most important thing to her IMO.
I think you are doing the right thing losing slowly and gradually - keep on, it's gruelling but good in the end I reckon. Good luck! 