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

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.

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