Personal experience: I started playing guitar in 2009 and didn't pay much attention to it before. 13, suddenly found that the right chest is smaller than the left chest. But I didn't think it might be the influence of practicing guitar.

I started going to the gym in March this year, and my chest muscles worked well. Then the size difference between my left and right breasts became more obvious. At first, I thought it was caused by irregular movements, but it was still the same after two months of adjustment. (In fact, it is normal that the size of the left and right breasts is different due to irregular fitness movements, but this happened before my fitness, which should not be caused by fitness. )

I didn't know it was caused by practicing guitar until last month. Because when playing the guitar, the right hand is raised and the chest muscles are stretched, over time, the right chest will be a little flatter than the left chest.

My humble opinion, I hope to adopt it