According to the Susan G. Komen foundation, women who are overweight or obese before menopause actually have a 10-20 percent lower chance of contracting breast cancer.
For one thing, weight is linked to a whole host of health markers, not just cancer risk. Improving your other health markers can reduce your overall mortality rate.
If you want to improve your health and reduce your risk of breast cancer, the best thing you can do for your weight is to maintain a healthy weight throughout your life.