I'm a A cup, 5'9, and 130 lbs. I want more than 400cc. My doctor has said I am too small to go bigger. Is this true?

As long as the diameter of the prosthesis does not exceed the normal anatomic limits of your breast any size of prosthesis can be placed. This is a simple arithmetic calculation. Different profile implants are available for women who desire more projection without the diameter chest wall for flatter profile implants.

Thank you for the question.  In regards to specific size/profile of breast implant, despite good intentions, online consultants will not be able to provide you  with  meaningful specific advice.  Ultimately,  consultation  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 are looking for. Ask to see lots of examples of his/her work.  2.  Again, 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 "C or D 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.  The use of temporary intraoperative sizers, viewing the patient's breasts in the upright and supine position, are very helpful during the breast implant selection process.
I hope this, and the attached link, helps.

Sizing of breast
implants is a very individualized
process. Essentially, you need to custom fit the implants to your body.
In order to properly size implants, you need measurements of several key

items. Regardless of what manufacturer of implant your surgeon uses,
they all have what is called dimensional fitting systems. These systems
basically take some measurements of your breast width, height, and
tissue stretch to determine the optimal volume for your particular
tissues. i.e. What can your tissues handle? That is the "science" of
determining the optimal size implant for your tissues. The artistry with
your surgeon comes in taking the ideal size and working with some
judgement to get you as close as possible to your desired outcome. So
someone 5'9" 130 lbs with
3 children vs. someone with no children may have different optimal
volumes due to the differential stretch in their tissues. You can
Google
"High Five Augmentation" to get a quick idea of the process.Without knowing precise measurements, it is difficult to say whether or not 400 cc is too large, too small, or just right. Good luck.