Sounds like your mom is just very insecure ... to be on the "nice" side, maybe she doesn't want you to have to deal with the issues she obviously struggles with.
I can still remember my Mom telling me that if I "sucked in my stomach" it would stay that way - I was 8 and going through the chubby stage (but not at all fat!!). My Mom is a very tiny sicilian woman and at 70 still does look great but has always been very concious of her looks and figure. I grew out of the chubbies and have been very lean up until I had my son around 11 years ago, then I struggled to stay in proper shape. Seems like we can never make them happy - too heavy, too thin, lose weight - but then you get saggy stuff. Really I think she's just trying to live through you. Make the decision it's your life, mind and body - and not hers. Tell her lovingly that you appreciate her concern (which I'm sure it's what it really is) but that you need to take this path on your own and do it your way. Good luck! Not sure if you have children, but as a Mom, I NEVER want my child to struggle with anything so it's hard to hit the balance between obsessive and positive guidance. Honestly, I realize that sometimes it's actually good for a person to struggle at some point as it teaches them coping mechanisms and makes them learn to solve their own problems ...
Sorry - I went on a little about that. It just hit home, from both sides of the story