Why would my surgeon give me smaller implants than we had agreed upon?

Hello, only your surgeon will be able to tell you why differently-sized implants were inserted.  I would suggest discussing your concerns with them.

I think you need to clear this up with your surgeon, rather than the theatre nurse. If you are not happy with the explanation then get an opinion from another local surgeon about what to do about it, if you are not happy. 
Regards, Dr Steve Merten, Sydney

Thank you for your question. I'm sorry to hear you are unhappy with your recent surgery. I suggest an in-person discussion with your board certified plastic surgeon to discuss the decisions made during your procedure. Hope this helps.

Thank you for your question. I am sorry to hear you are disappointed with breast augmentation result. Please discuss your concerns with your plastic surgeon.  Typically if a plastic surgeon uses a smaller implant during the surgery than was discussed, it is because the larger size does not fit the body. It is not unusual to switch implant sizes during the procedure to get the ideal fit for the patient. The main question should be whether or not you are pleased with the result with less focus on the actual size of the implant.  

Best to have a chat with your surgeon as they will know what took place during your procedure and why different implants were used.

I am sorry to hear about your disappointment after breast augmentation surgery.  The only honest response online consultants will be able to provide you is:  I don't know.  Only your plastic surgeon, who was there and worked with your tissues, will be to provide you with  a meaningful response.
Although I cannot help you with specifics, it is true that every patient undergoing breast augmentation surgery has an anatomic starting point will limit the outcome of breast augmentation surgery.    In other words, a patient's starting anatomy will limit how large of a breast implant can be utilized safely. The use of a larger breast implant (than what is safely "allowed" by the patient's anatomy) can be problematic;  potential problems include breast implant displacement/malpostion issues ( such as bottoming out, lateral displacement, symmastia...)  and/or significant breast implant rippling/palpability.
 Generally speaking, breast implants will "drop" into the pocket that was created during surgery. The rate of breast implant "settling" may depend on factors such as size of breast implant pocket dissected, tightness of the overlying skin/muscle layers, and size of breast implant utilized. Normally, it takes about 3-6 months (for some patients longer) to see the final result. 
Many of my patients report that as the swelling around the breast implants (for example, in the sternal area) decreases, they feel that their breasts appear more projecting (and that they have better "cleavage"). Also, in my opinion, the "drop and fluff” phenomenon does hold some validity; as breast implants “settle”, many patients feel that their breasts actually look larger. Similarly, it takes a few months for the breasts to "soften" in most cases.  Most of my patients tell me that their breast implants feel "part of them" somewhere between 3 to 6 months after surgery was performed.  There is definitely a physiologic and "psychological" (body image)  adaptation process that occurs, at differing rates for every patient, for patients who undergo any type of aesthetic surgery, including breast  augmentation surgery.  Understandably, patients often experience significant emotional "ups and downs" around the time of surgery.
If in the longer term you remain displeased with your breast size, re augmentation with larger breast implants may be an option.
 Best wishes for an outcome that you will be very pleased with.

Your surgeon was trying to give you a look that would probably look natural and use a size that fits your frame and tissue. Using implants that are too large will often give poorer long term results, some which can be difficult to fix. I would suggest asking discussing your concerns with your surgeon.