Will weight loss and being thinner help someone live longer with breast cancer?

Perhaps IF not due: the breast cancer or treatment itself, because this works in general. However, in this situation, weight loss & thinner commonly a bad sign because the individual's health is powerfully being degraded; both the cancer & the treatments typically used. Most cancer treatments are not well targeted at the cancer cells, thus all the side effects. So how dose most Tx work? Healthy cells recover better.