Talk to Gynecologist on Flat Breats

I am 28 years old woman with history of PCOS. Have been under treatment for it on and off for five years. The issue is my breasts have gone flat over time. I am an underweight person and have been trying to gain weight but nothing works. I want suggestions on increasing weight through diet and exercise, no medicine of any sort. Also, I want to know if I would be able to increase breast size? any recommendations for it?