Are 400cc and 425 cc Siltex/Mentor HP gel implants, placed dual plane, too big for my breast width? (photo)

Although it is difficult to tell with photos alone- that sounds about right. Can't imagine you'd be too big although cup size is fairly subjective. Here is no one method that is 100% reliable in arriving at the optimal size . Preop sizing and imaging are helpful but do not necessarily predict cup size or satisfaction. This is a challenge to all surgeons. I don't think anyone on his site would argue that selection. ( ok maybe Dr Tebbetts!)

Thank you for the question and pictures. Base on your photographs, you are starting at a great place and should have a very nice outcome with breast augmentation surgery.  However, despite good intentions, online consultants will not be able to provide you with  meaningful specific advice.  Ultimately, in person consultation with well experienced board-certified plastic surgeons will be necessary. Careful measurements, dimensional planning, and careful communication will be key.
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're 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 which operation and/or 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 "DD cup" etc 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.  The use of computer imaging may be very helpful during this communication phase.
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.
I hope this, and the attached link, helps.

I think that a simple test for you to do is a baggy test with the volume you would like to see if you are comfortable with that volume.  Good luck.