How Does Sizing Affect a Low Profile Implant?

I rarely use the lowest profile implants in silicone. You are right, that under the muscle you will loose some projection. Silicone implants in the appropriate size for your frame typically look natural. It is important to pick an implant diameter that fits within the width of the breast. If it is too wide (wider than your breast width), it could stick out a bit laterally. I would trust that your doctor is taking accurate measurements to assure that you are choosing an appropriate size. Sometimes the narrower, midrange profile implants do look more petite and suit patients who are narrower, while maintaining naturalness. 

Low profile implants are best used in women who have a broad chest, and don't want a lot of projection or a fuller size . Lo profile implants tend to be wide and are uncommonly used. Without knowinf the specifics of your case, that is your height weight current bra size and chest dimensions, it is difficult to comment on your specific needs.