It would certainly appear by your picture that a smaller dome would be useless. Actually I think you should consider the larger XL. I have a couple of theories, and to start with, why the pain? The CLs are generally considered to be more comfortable than the other styles, where is the pain, at two points or spread around the dome edge? The contoured large cups are contoured to fit the rib cage better, that is the curve goes around the chest. Do you have them rotated to the most comfortable position? Are you using a generous coating of oil like safflower oil (in your grocers salad oil section)? Are you massaging the skin where the dome presses between sessions? The only reason I mention the XLs is because if you alternated, the larger diameter would not press on the same spot, thus giving some relief. Another possibility is to move the domes to slightly different positions (like lower or closer together) to move the pressure points around.
I also wonder it you are using excessive pressure, not likely since you are not complaining of red dots (broken blood vessels) or bruising, but my other theory is that best results might be attained by pumping hard and long (many 7 minute sessions) one day a week and gently for the rest of the week. The idea is to apply just enough pressure during the gentle cycle to create mild swelling but not stress the body, and try for maximum swelling once a week. The gentle cycle should be completely painless and allow recovery.
Best of luck