Large Breasts can be made smaller for both COSMETIC and FUNCTIONAL reasons.
IF your breasts are a bit larger and saggier than you would like them and you would prefer smaller but perkier breasts, your Plastic surgeon can reduce a variable amount of breast tissue from each breast to make them more alike, reduce their size and make them perkier. As a cosmetic procedure you will pay for this out of your pocket.
When an insurance company reviews your complaints of breast, shoulder and back pains which are NOT relieved by non-surgical means and approves a (FUNCTIONAL) breast reduction, it would often stipulate just how much breast tissue must be removed from each breast (usually at least 500 gram). If this amount is not removed, it is within its rights to refuse paying the surgeon, anesthesiologist and facility for your surgery, making you fully liable for all these charges.
The risk of going from a full C to a full A comes when a woman whi should pay out of pocket for a cosmetic breast reduction goes the route of having it done through insurance. Done this way, the surgeon must remove the base amount demanded by the insurance.
It is best to discuss YOUR specific case with your surgeon and see which would be the best way to go especially if you are a smaller woman.