Jane, it's impossible to say if your weight had anything to do with your current cancer.  The theory of why being overweight is a breast cancer risk factor for post-menopausal women is that fat stores estrogen and too much estrogen after your body has gone into menopause is a risk factor for breast cancer.  However, as you point out, your cancer doesn't feed on estrogen, so your weight might have had nothing to do with it.  Even if it did, there is no point in beating yourself up about it now.

You'll want to talk to your doctor about nutrition during chemo and radiation; however, I don't think this period of stress is an ideal time to start a calorie-restricted diet.  Chemo is going to affect how food tastes, and you may be queasy with certain foods. One important step in life style you could take would be exercise.  There is good data that exercise helps prevent recurrence of breast cancer, and it also helps with depression, weight, and energy.  Consider starting with very short walks and then gradually build up.

Other breast cancer risk factors that you can control are drinking alcohol and smoking.

I wish you the best as you go through your cancer treatments.