I'm 5"8, weigh 128 and a 36A, are 475-500cc silicone implants under the muscle to big?

Thanks for your question. It's difficult to assess without the benefit of your pictures. IMHO 475cc-500cc may be too large for your current size. I don't know if you've had kids breast fed etc. All of which would help in determining how much skin laxity and soft tissue coverage is available.I would definitely use silicone gels placed behind the muscle. If you use saline, the chances of rippling or wrinkling go up exponentially as you increase in volume. The other question you need to ask yourself is what look do you desire. Fake or natural? Concentrate more on the look than the actual eventual cup size. Always remember that the eventual cup size following a breast augmentation is always the sum of the existing breast tissue volume plus the added implant volume. The same size implant in you may be a totally different cup size down the road than your friend.I have been in practice and performing breast augmentations for the past 29 years in the land of breast implants, Miami, Florida. I feel that many of the augmentations that I see have been overdone and look fake and vulgar. I prefer a natural look to an enhanced breast. The best compliment a patient of mine could be paid is: " I would have never guessed you had breast implants". Women have very creative ways of showing off their new breasts. However, what is appropriate for a social function or club is not appropriate for work.Think about it. When and if you decide to have your surgery please seek out a surgeon certified by the American Board of Plastic Surgery with years of experience in breast augmentations.How about the center, clinic or facility? Are they accredited by a national organization or do they just have State approval. Understand that at the current time, there are three nationally recognized organizations responsible for the highest levels of patient safety, AAAASF, AAACH and JCHO. You owe it to yourself to position yourself for the best possible results but under the most stringent safety regulations, If you have kids, even more so.How about anesthesia? Will you have a medical doctor certified by the American Board of Anesthesiology or a certified nurse anesthetist (CRNA)? Understand that there is no substitute for research. Cosmetic surgeries, no matter how simple they may be to the patients, are invasive procedures and as such carry certain risks and complications.I wish you the best on your journey. Good luck

Thank you for the question and pictures.  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 "C or D cup" or "busty look" or "silly" 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.