It is believed that being overweight or obese after menopause increases breast cancer risk. Before menopause your ovaries produce most of your estrogen, and fat tissue produces a small amount of it. But after menopause (stopping of monthly periods), most of your estrogen comes from fat tissue. Having more fat tissue after menopause can increase your estrogen levels which can increase your chance of getting breast cancer. In addition, women who are overweight tend to have higher blood insulin levels which have been linked to some cancers, including breast cancer.
The link between weight and breast cancer risk appears complex. Since, the risk appears to be more in women who gained weight as an adult but not for those who have been overweight since childhood. It also depends on area of fat deposition. Excess fat in the waist area may have more risk than the same amount of fat in the hips and thighs.