Do you think 300cc is too small at 5'1 and 121lbs? (Photos)

Congratulations on having undergone the breast augmentation procedure. Although online consultants will not be able to provide you with meaningful reassurance, it may be helpful for you to know that a significant percentage of patients at your stage of recovery will feel that they are too big or (more commonly) too small. I routinely ask my patients to wait at least 3-6 months before evaluating the end results of the breast augmentation surgery. This waiting time allows patients to (usually) physically and psychologically adapt to the new body image. 
From the physiological standpoint, it takes many months ( and even up to one year) to see the final results of surgery. factors such as swelling, tissue contraction, skin redraping, implant shifting etc. come into play. You may be pleasantly surprised wants to be "drop and fluff" phenomenon has occurred. You may also be happier once the swelling AROUND the breast implants dissipates and the implants' shape and projection become more visible. 
From the psychological standpoint, patients undergo an “adaptation period" during which time they get used to the changes that the surgical procedure I would send has brought about. Severe emotional swings can also occur after any type of surgical procedure.
Although I cannot comment specifically about your anatomy and its limitations, (generally speaking) every patient undergoing breast augmentation surgery should understand that their 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.
In your case, I would suggest continued patience and continued close follow-up with your plastic surgeon.
Best wishes for an outcome that you will be very pleased with long-term. 

You need a formal evaluation by a licensed plastic surgeon and expert in breast augmentations to determine what is best for you and your goals.
Proper breast enlargement is not a "cookie-cutter" procedure where the same technique is applied to every patient. Every woman who comes to my practice has unique needs and different goals for her breast augmentation. As a board-certified plastic surgeon, I always listen carefully to each woman's concerns and thoroughly discuss the key points of the procedure. I also take into account their overall height and weight, the size and shape of the underlying chest and ribcage, the tightness and elasticity of the skin, and the amount of natural breast tissue. Prior to the procedure, we discuss and make decisions regarding location of the incision, whether to place the implant under or over the chest muscle, and what type of implant to use (I offer multiple types of silicone gel breast implants, as well as saline implants).
My experience performing thousands of aesthetic breast surgeries in a wide anatomical range of patients enables me to achieve consistently beautiful and natural results that are customized to each patient’s body type and goals. These results are the product of extensive communication, meticulous planning, and skillfully executed surgery.
I suggest consulting with a board-certified plastic surgeon and expert in Breast Augmentation surgeries. - Dr. Cohen

Dear Maisly1995,
I understand your concern and it is quite normal to be concerned about the outcome following a breast augmentation surgery. However, you are still in the very early stages of the healing process.Your will need to be patient at this moment and follow what your surgeon has instructed you to do post operatively.At the early stage of the healing process, the tissues around the implants are still swollen. Once the swelling subsides and the implants drop to their final position, you will be able to see the final result of your breast augmentation surgery.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery