Should I go with 450cc to get the 425cc sizer look?

Factors which contribute to the outcome of Breast Augmentation are #size, #shape, and #profile. The #profile is determined by the amount of #projection versus roundness that the you desire, as well as the #size of the implant and size of your chest. Also, the placement or incision location will also have an effect on profile. This may also provide some insight. If a woman wears a size 36 AB bra and would like to be a C cup, which is the average size that women wear and the average size bra that is sold, she needs a volume of approximately 350 to 390 cc to make that change. This becomes less if she's wearing a 34 bra, because it has a smaller strap size and the cups have a smaller diameter. A smaller diameter implant would then be a volume of approximately 300 to 350 cc. In the same way if she was more petite, maybe a double zero or size one and wore a 32 AB bra, then to become a 32C cup she would only require about 250 to 300 cc. The profile is determined by the amount of projection versus roundness that the you desire, as well as the size of the implant and size of your chest. As much as it's important to know what size bra you currently wear, the strap size is important to know as well. It affects the diameter of the cup and subsequently the diameter of the #breast implant that you choose. In my practice, about 15-20% of the patients are good candidates for silicone #shapedimplants.#Subpectoral or #submuscular implants refer to placement under the muscle which still remains the most common placement in the United States. #Submammary or #subglandular implants are the placement under the breast tissue in front of the muscle. This tends to more common in Europe and South America. General guidelines suggest that women with larger drooping (ptosis) breasts that do not want a breast lift often have the implant placed above the muscle, although another option, #DualPlane, has become more popular. In the Dual Plane method, the implant is placed under the pectoralis muscle but the breast tissue is also release, in part, from the muscle. Those patients with minimal breast tissue should usually have the implant placed beneath the pectoralis muscle for more soft tissue padding.

25cc is unlikely to make a significant difference in the appearance of your breast augmentation. It's a question that's best directed to your surgeon, but in general I recommend that patients who are undergoing a submuscular implant go one size up from the sizer that they like in the office. The muscle can flatten the implant slightly and make it appear smaller.

Thank you for the question.  Be careful that you don't “overthink” breast implant sizing concerns ( keep in mind that the “sizers  look" does not necessarily translate into the breast augmentation look achieved). Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, and computer imaging) will be critical.  
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural” or "C or D cup” etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate.  Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery,  after the use of temporary intraoperative sizers.
I hope this (and the attached link, dedicated to breast  augmentation surgery concerns) helps. Best wishes.