How big do you think a 520 cc silicone implant under the muscle will be? I am 5'7" and about 120 lbs. (Photo)

A 520 cc implant on your size is going to be really big. Probably in the DD to DDD range. So, make sure that  this is the look that you want. Also, keep in mind that really large implants will definitely cause your breast tissue to sag over time. Best, Dr. Nazarian

A 520 cc silicone implant under the muscle for someone who is 5'7" and 120 lbs will likely result in a significant increase in breast size, likely reaching a DD to DDD cup (keep in mind cup sizes are subjective). This is considered a large implant, especially for a smaller frame. It’s important to note that the final look can vary depending on factors like chest/breast width, existing breast tissue, and skin elasticity.
While 520 cc implants can provide a dramatic transformation, it's essential to consider the potential long-term effects, such as the risk of breast sagging due to the added weight. Some people may also experience discomfort or back pain, especially with larger implants. To ensure the best outcome, it’s important to discuss your goals thoroughly with an experienced plastic surgeon, try on sizers to visualize the result, and consider using tools like 3D imaging to predict the final outcome.
Krishna Vyas, MD, PhD, MHSHarvard, Hopkins, & Mayo Clinic Trained Aesthetic Plastic SurgeonNew York, New York

A 520cc implant on someone your size will be a very dramatic change. They will be large, and they will look like augmented breasts, so be sure that that is your goal! There is no direct correlation between implant volume and cup size, but I would guess this would put your cup size in the DD-DDD range. Assuming your base width is in the 12-13cm range, this will likely mean using an extra-full profile implant (or whatever the equivalent is in your surgeon's chosen implant)