Most Doctor who specialize in the research and treatment of Macromastia tend to agree that the body can support the weight of larger breasts than is typically found in most reduction surgeries, without pain. They say the pain is often caused by the woman wearing the wrong size, or improperly fitted bra for her size. After all, studies show that over 70% of women are wearing the wrong bra size.
If you are a large breasted woman, and you have straps digging into your shoulders, which is causing back pain, you are wearing the wrong size of bra. A properly fitting bra should not be placing any real weight on the straps at virtually any size.