I have said this to so many patients over the 24 years I have been doing this surgery, but there is a reason it needs repeating over and over! 50cc is 3 tablespoons and 1 teaspoon of volume. Put that volume in a glass and try to visualize how much that volume over an entire breast will increase it in size. Barely noticeable at all!
75cc will be (slightly) visible, but, by my rule of estimating, adds less than 1/3 cup size to whatever breast tissue you have now. This is a critical fact. If you simply use 350cc and increase it to 400cc, this is indeed a 14% increase. But your breasts consist of your own tissue plus implants, not just implants alone! Let's say you have only 250cc (about one cup size) of your own tissue to which the 350cc (600cc total) is increased to 400cc (650cc total). This is only an 8% increase in size! If you do the math for the 425cc implant, the increase (again assuming you only have 250cc of your own tissue, about an A cup) is only 13%. Either of these increases is indeed an increase, but so minimal you would hardly be able to see a difference! If your breasts pre-implants were somewhat larger than A cup, the actual percentage of increase is even smaller!
As others have said in their answers, you will definitely need more volume to see a visible change. I would argue that it takes about 250cc to equal one cup size, and this is based on thousands of these surgeries. Since I do not charge a surgeon's fee for re-operation (patient pays for OR, anesthesia, and new implants) this is a LOSE-LOSE for both surgeon and patient when size is wrong, so I truly believe this is good advice, and respectfully disagree with my colleagues who are more conservative. Perhaps they charge for re-dos, or work in a university setting with a salary, and have less of an incentive to "get the size right." My reputation and income depend on my having as few do-overs as possible, even for size change!
At any rate, you might want to read my article (on the "About" tab on my Profile page in this site) titled: "What is the Right Breast Implant Size for You?" for more information. Your hunch is probably right here.
As an aside, why does your surgeon have to order implants, constraining you to the choices made before surgery, when the best choice in the operating room may in fact prove to be a larger implant? What then? Later re-operation and a re-order of the "correct" size? Who pays for this? Many surgeons who do lots of breast implant surgery have a full inventory of breast implants on-site (if they operate in their own surgical facility as I do). I have every size choice and profile available in the operating room. Sterile sizers can be used to determine the proper size for your post-baby anatomy and your goals for the surgery, and any intra-operative findings that might change your surgeon's mind can be taken care of without the excuse of: "That was all we had available!" Even hospitals and surgicenters have a larger inventory than two choices! (Both of them wrong, IMHO).
Kyla, listen to your gut! You probably need 500cc or larger to really make a difference. Best wishes!