wrt laxity caused by bras (from wikipedia entry about Cooper ligaments):
it seems that pectoral muscles will effect the look of cleavage, but i don't see how they would make your breasts less saggy.Dr. Christine Haycock, a respected surgeon at the New Jersey Medical School and an expert in sports medicine, said that "Cooper's ligaments have nothing to do with supporting breast tissue... They just serve to divide the breast into compartments." She noted that most women's breasts begin to droop with age and that extremely large-breasted women are generally more affected. However, sagging is not related to ligaments or dependent on breast size.