She is besieged every day, as are all our nation's girls and women, by images of females that tell them they must be thin to be appealing. These images have created an obsessive fear of being fat and an overwhelming number of girls with eating disorders.
Your daughter knows the difference between a Twinkie and a skinless piece of chicken. If she is eating too much junk food or just plain too much food, it's not because she is hungry. She is using food for other psychological/behavioral reasons -- anxiety, self-medication, to combat boredom, as a mindless activity to do while watching TV, etc.
Of course there is a genetic component that plays a role in determining body size, but that factor is not the most significant in determining whether kids are fat or thin. A healthy diet and getting regular exercise are the two most significant determinants in anyone's body size.
I would suggest that you check out your own issues concerning your body size, food, and obesity. Your worries impact your daughter's thoughts, emotions and behaviors regarding her weight. She should not be "at war" with her growing body. You can help her by apologizing for pressuring her in this matter. It does not matter how well meaning your "watching her weight" has been. It has hurt, shamed, and angered her. She needs to know that her size will never affect whether you love her or appreciate her.
Make sure that your home has healthy foods in it but don't make it a "fat-free zone." Model healthy eating and exercise behaviors without drawing attention to yourself in the process. If you feel that your daughter is reaching out to you for help in this area, despite her expressed anger and sadness, simply ask her if there is anything that you can do to support her.
It might be worthwhile for you to consult with a therapist who focuses on these teenage body image/obesity issues. There are specialists in this area who cover all aspects of the problem -- family dynamics, nutrition, individual emotional issues. Please also do a word search on our site as I have written more extensively on this matter and we have provided a list of helpful resources as well.
I am sure that you can heal the sadness and separation that is currently felt. This is an especially sensitive area, as you well know. Be compassionate, listen to her and show her that you love her unconditionally.