It's normal for adolescent girls to go through a period of relative chubbiness as they go through puberty. The hormone changes that occur, with higher estrogen levels present, allow the body to put on more fat. This is what is responsible for further growth of the breasts and rounding of the hips. Over the two years after a girl starts her period, growth in height stops and the body remodels itself further. This is when many girls will slim down again and get to a weight that is more appropriate for their height.
It's important that you make sure you are not overreacting to this normal plumpness. Girls who are made to feel that they are "fat" during adolescence may end up constantly feeling that they are overweight. This in turn can lead to eating disorders or just poor eating habits.
I think it's important for you to help her make healthy choices for her foods, but without isolating her as the only target. That is, you should encourage the whole family, not just your daughter, to make healthy food choices. You can do this most effectively by modeling healthy food habits for her, and keeping the snack foods out of the house for everyone. You should also make sure she is getting enough calcium and not drinking a lot of soda and soft drinks -- at least while she is home.
More importantly, you also want to make sure your daughter is engaged in some form of regular exercise each week. By this I mean more than just one or two physical education classes a week, but some sort of active exercise at least 4 times a week. This can be a sports team at school or in your town, swimming at the local pool, dance or other exercise classes, or just walking or riding a bicycle for 30 or 40 minutes. Children who get into the habit of regular exercise by the teenage years are more likely to continue to exercise as adults, and thus are less likely to become overweight as adults.