John Smith:
The only way for a heavier woman to reduce breast pain is to either losing weight and hope her bust size is going to decrease along enough to creep below D cup territory, or working out extensively and buff out so that her bust size gonna deflate because of pectoral muscle enlargment.
Or, if she's really buxom even after some severe weight loss, she could envision to pass a breast reduction surgery.
Or... an another eventuality would be to find a way to mentally cope up with the pain.
I’d like to respectfully disagree with this. Breast tenderness and/or pain is most often linked to hormonal factors, and age. If your gf is in her early-mid 20’s or younger, it’s entirely likely that she’ll grow out of it. If it’s uncomfortable enough that she’s willing to seek help for it, a conversation with her GP or gynaecologist should be her first stop. Following a physical examination (to make sure her pain isn’t associated with tumours, basically), they’ll likely discuss hormonal birth controls as options, which can help moderate estrogen and progesterone levels.
*Disclaimer: I’m not a doctor, but I have worked in reproductive health!*