Downsizing breast implants
This has nothing to do with your frame or chest characteristics. Unfortunately one cannot just pick a size (volume in cc's) of implant and make it work. Breast implants need to be properly fitted for the space behind the breast or in the case of existing implants, the capsule around the existing implant.
Because the capsule space will not shrink down reliably and it should not be narrower than the breast width anyway, the replacement implant needs to be the same diameter or only...