Why won't my surgeon reduce my breast size to a B cup?

Size after breast reduction is somewhat difficult to predict in many cases, as patients often do not have a clear understanding of what it means to go down to a size like a B cup. Women who are accustomed to very heavy, large breasts or who are overweight may feel their breasts are too small after a reduction, even if the cup size was "agreed upon" (there should never be a direct explicit guarantee of a postop breast size) and matches the bra that the patient can obtain postop. If the volume is inadequate to support reasonable cleavage, and especially if nipple blood supply is compromised, it does no good to have an arbitrary cup size if the breast doesn't generally look good or there are major healing problems from nipple loss. It is easier to further reduce a breast than to add volume back, so your surgeon may be thinking along those lines based on experience.

You should be able to find a surgeon that will work with you on this, especially since you are paying for it.  You already are quite informed about the procedure but aggressive resections can be done to 'approximate' your results.  If you remain larger than a desired B cup, realize revisions will be needed so know your surgeon's revision policy.  If your surgeon refuses to accommodate your desires, there are plenty of others that would be happy to have you as a patient.