Many women implanted with saline implants remark that they like the appearance of their breasts, when they are recently done. While they all prefer the feel and mobility of the breasts after they descend, some experience "buyer's remorse", like you and wish they had chosen larger volumes. Discuss your dissatisfaction with your surgeon. If your implants are filled to the minimum volume, there is some discretionary "overfilling" of the implants, which can be done, however it is usually in the 25-50cc range, depending on the basic volume of the implant and is specified by the manufacturer.
If you and your surgeon agree to "overfill" the implant there are several risks which include: infection, worsening of the external and/or internal (capsule) scar, damage to the implant, and dissatisfaction. When a saline implant is overfilled, it may feel firmer and while the projection may increase, the space between your breasts may widen, as the footplate becomes narrower. On the otherhand, switching to a larger size breast implant which can easily accomodate the desired 100cc volume while more expensive, is probably the most reliable method to achieve your aesthetic results. Discuss this with your surgeon.