Will 375cc HP be too small?

Thank you for the question. Based on your photograph, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery. 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, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) 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. For example, I have found that the use of words such as “natural look” for "small 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 for an outcome that you will be very pleased with.

you found a thoughtful and safe conservative surgeon that is a plastic surgeon to boot given that he or she is using Sientra. Yes you could fit more, no you will not get a D with 375. There's probably more skin stretch and a more experienced surgeon would be able to tell you for sure that more would fit, but having a great result that isn't botched is way more important than just going big. Definitely speak to them, though, and share your concerns. Larger sizers might be a way for them to check and see what size might work best. I don't use sizers and simply go on breast measurements but there are lots of approaches that work. Best of luck. 

A good examination is critical to help determine the best size range for you.  Then sizing with sizers helps to see how it the volume will affect the body symmetry.   If also helps you see what you will look like in the future.    If you still feel small at a certain size, you may be unhappy with the results in the future.    If you are questioning yourself, you may need to get a second or third opion for other surgeons.