Many studies have shown that overweight and obesity are associated with a modest increase in risk of postmenopausal breast cancer. This higher risk is seen mainly in women who have never used menopausal hormone therapy (MHT) and for tumors that express both estrogen and progesterone receptors.
Overweight and obesity have, by contrast, been found to be associated with a reduced risk of premenopausal breast cancer in some studies.
The relationship between obesity and breast cancer may be affected by the stage of life in which a woman gains weight and becomes obese. Epidemiologists are actively working to address this question. Weight gain during adult life, most often from about age 18 to between the ages of 50 and 60, has been consistently associated with risk of breast cancer after menopause.
The increased risk of postmenopausal breast cancer is thought to be due to increased levels of estrogen in obese women. After menopause, when the ovaries stop producing hormones, fat tissue becomes the most important source of estrogen. Because obese women have more fat tissue, their estrogen levels are higher, potentially leading to more rapid growth of estrogen-responsive breast tumors.
The relationship between obesity and breast cancer risk may also vary by race and ethnicity. There is limited evidence that the risk associated with overweight and obesity may be less among African American and Hispanic women than among white women.
Many more details at - http://www.cancer.gov/cancertopics/factsheet/Risk/obesity