Bench pressing doesn't decrease breast size. However, if excess fat in the breasts and back is burned off, it reduces bra size.  Bench pressing doesn't increase breast size. However, the muscles that sit under the breasts are the pectoralis major and minor, and they can grow in size; when this happens, it creates an illusion of breast growth, but in reality, it is the pecs that are under the breast. This also causes the breasts to sit perkier on the pectorals.