I can't help but wonder where you've got this information from?
Neither pectoralis major nor minor have any significant effect on breast size or 'perkiness' - whilst some of the breast tissue is attached to the overlying fascia, the shape of the breasts are largely determined by intrinsic connective tissues and the breast tissue itself. Toning of the pectoralis muscles may serve to slightly elevate the breast overall, but will have negligible to no effect on the breast shape.
The notion also the a brassiere would somehow relieve strain the pectoralis muscles is also rather strange in itself. Pectoralis major adducts and flexes the humerus, and has little to no function below on the chest; whilst it is an accessory muscle for inhalation, this mechanism of which relates to pulling the sternum and clavicles upwards, not by any attachment to the breast tissue. Its prominence with exercise, addtionally, is simply the result of growth of the muscle tissue.
Whether the breasts are supported or not will have minimal impact on even the inhalation function; the 'extra' weight is still attached over the shoulders and requires the same energy to lift; and when lying down in bed this would be virtually non-existent as the horizontal nature of most sleep means that the pectoralis component of inhalation is not opposing gravity.
Or maybe I'm working on poor textbook anatomy and you've seen a paper which explores new roles of pecs re: breast shape?