I wear sleeveless or strappy tops and you can't tell much shoulder difference unless you are looking for it. There is a slight depression in front, where the frontal rotator cuff muscles are gone and a thin scar, but the rest of the shoulder is close to the same size.
I have had good arm strength and muscle due to daily work, so did not have much trouble returning my shoulder to full functionality after the rTSR. Since the frontal rotator cuff is removed, you do have to work with PT to build up the remaining muscles to compensate and take over some of the rotator motions. Certain movements won't feel as efficient.

I can believe that if post surgery is painful and the patient avoids or does minimal exercises, the muscles can shrink from dis-use and the whole arm appears smaller. Perhaps there is skill of the surgeon involved also in working with the remaining muscles and tissue.