What creates upper pole fullness?

It's a great question. All properly sized breast implants will create fullness in the upper pole of the breast. That being said, upper pole fullness is affected by the size of the implant, the profile of the implant (implant projection), and the shape of the implant. Round implants have more fullness of the top, while shaped implants have a tear drop appearance that has less fullness of the top of the implant. A careful explanation of your goals to a board-certified plastic surgeon will be helpful. They will help you choose the right implant.

Thank you for the question.
 
There are MANY variables  involved when it comes to the aesthetic outcome achieved with breast augmentation surgery. For example, when patient's wish to achieve significant upper pole volume, factors such as positioning of breast implant "pocket"  along with selection of appropriate breast implant type/size/profile are all important.  Of course, a specific patient's anatomic starting point will play a significant role as well.
I prefer to have a wide selection of implants ( all profiles) available in the operating room. I use intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
 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 “volume on top”  or "C or 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. 

The patient's anatomy contributes some but most of it comes from the profile of the implant used. High and ultra high profiles make the most upper pole fullness. The tighter a patient is built the more fullness they will retain.