There is no correlation between weight and breast cancer in premenopausal women.  After menopause excess weight is a risk factor.  One theory is that because fat cells store estrogen, overweight postmenopausal women have more estrogen that can feed a breast cancer.  However, your weight did not cause your cancer.  Many women with lots of risk factors for breast cancer never get it, and others with no risk factors whatsoever do.  Don't waste time worrying about possible causes of your cancer.  

Do consider what you can do to prevent a recurrence.  There is evidence that regular exercise reduces the chance of recurrence, so maybe you could start an exercise program that would help reduce you chances of having the cancer come back with the added benefit of possible weight loss.