Implants Too Small After Breast Reconstruction Surgery

I largely agree with most of the preceding comments and would like to amplify on them. Please do not take my comments as a personal criticism - they are NOT intended as such.
You have a potentially fatal disease. The duty of your surgeons is to exercise their best experience to separate that disease process from you and give you a long, recurrence free life. The mastectomy process itself largely determines what the plastic surgeon can or cannot do.
As Plastic surgeons we would LOVE it if our breast surgeon colleagues would allow us to mark their incisions for them to assure the scars will not be oblique but transverse and hidden in a bra, if they left as much breast skin as possible, if they made the skin flaps even a little thicker and not paper thin (and practically devoid of a stable blood flow, if they left the under the breast fold intact (making breast reconstruction easier and more natural) and if they could refrain from radiating the breast after the surgery permanently damaging our reconstructed breasts. Sometimes these requests are followed, often they are not and when we walk in to do the reconstruction many "bridges" have been burned for us.
Building a breast is NOT a matter of just picking and plugging a set volume implant into a matectomized chest. It calls for a LOT more artistry, experience and knowing when to accept good as good enough and NOT push further and lose everything with an overly large implant which can exert too much pressure on thin breasts flaps or a weakened infra mammary fold.
Discuss this with your surgeon and see what if anything can be done. Maybe if you understood his/her reasoning you would appreciate what you have.

The ultimate size of the breast after mastectomy and tissue expansion will depend on many factors other than the size of the implant.
Tour weight, height, and fat distribution will play a role in the final cup size.
A 450cc implant is a good size implant, going larger will depend on the amount of tissue cover for the implant.
Remember the larger the implant the more frequent the need for further surgery.
Le the present size settle down for a while then make a decision regarding the size of the implant. At that time discuss with your PS your desires and size, discuss the pros and cons of increasing the size of the implant, fat distribution especially on the lateral chest that may cause the breast to look smaller.