Is 425cc Too Big for B Cups with C Cup As Goal Size?

No one can answer this question for you.
Your best option, in my opinion is to try on sizers, give your input, and take responsibility for your decision. What works for your lifestyle now may not be desireable 5 years from now and that means either larger or smaller. Your surgeon, is making a recommendation based on his/her 20 year experience. He/she likely knows that the average person desires a larger implants after surgery. However, you may not be the average person and he/she cannot read your mind. Express your thoughts, desires, and goals.

The more breast surgery I do the more I realize that there is no correlation between the size of implant and resulting cup size.  This may have to do with several factors including: the amount of breast volume the patient starts with, the shape of the patient's chest wall (concave or convex), the type and model of breast implant selected (saiine/silicone  and low/moderate/high profile), bra  manufacturer variance  in cup sizes, the  degree  of filling of the cup  with breast tissue,  and the subjective differences in patients perceptions of cup size. 

Much of the final “look” achieved after breast augmentation surgery  depends on several factors:
1. The initial shape, size (volume of breast tissue), symmetry of the patient's breasts. In general, the better the  preoperative breast appearance the more likely the breast augmentation “look” will be optimal.
2. The experience/skill level of the surgeon is important in determining the final outcome. For example, the accurate and gentle dissection of the breast implant pockets are critical in producing  long-term  well-placed breast implants. I personally think that these 2 factors are more important than any others, including type (saline or silicone)  or model (low/moderate/high profile)  of implant.
3. The type of implant used may  determine the final outcome, especially if the patient does not have significant covering breast or adipose tissue. For example, some surgeons feel that silicone implants have a more natural look and feel than saline implants because silicone gel has a texture that is similar to breast tissue. Each patient differs in the amount of breast tissue that they have.  If a patient has enough breast tissue to cover the implant, the final result will be similar when comparing saline implants versus silicone gel implants.  If a patient has very low body fat and/or very little breast tissue, the silicone gel implants may provide a more "natural" result.
On the other hand, saline implants have some advantages over silicone implants. Silicone implant ruptures are harder to detect. When saline implants rupture, they deflate and the results are seen almost immediately. When silicone implants rupture, the breast often looks and feels the same because the silicone gel may leak into surrounding areas of the breast without a visible difference.  Patients may need an MRI to diagnose a silicone gel rupture.   Saline implants are also less expensive than the silicone gel implants.
Other differences involve how the breast implants are filled. Saline implants are filled after they’re implanted, so saline implants require a smaller incision than prefilled silicone breast implants.
On May 10, 2000, the FDA granted approval of saline-filled breast implants manufactured by Mentor Corporation and McGhan Medical. To date, all other manufacturers’ saline-filled breast implants are considered investigational.
As of 2006, the FDA has approved the use of silicone gel implants manufactured by the Mentor Corporation and Allergan (formerly McGhan) for breast augmentation surgery for patients over the age of 22.
4. The size and model of breast implant used may  make a  significant difference in the final outcome. Therefore, it is very important to communicate your size goals with your surgeon. 

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 cup” or "fake looking" or "top heavy" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
I use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
By the way, the most common regret after this operation, is “I wish I was bigger”.


I hope this helps.

For your body and frame that does sound a bit too big. But it is really hard to say just from a bunch of numbers. One would need to see you and do base diameter measurements to see how wide your chest is.
Also what you can do is to perform the rice test to see what 425 cc looks like on you. Here is a journal entry on the rice test. http://www.feplasticsurgery.com/journal/orange-county-breast-augmentation-sizing/66/
Hope this helps you out.