All breast augmentations that I perform are subpectoral (under the muscle). After surgery, patients look excessively full at the top of their breasts because the muscle tightens after surgery, and the implants take time to "settle" into place as the skin relaxes and stretches and the muscle also stretches to accommodate the added volume to the chest. The body forms a "capsule" around the implants which is a thin layer around implant (which is a process the body does to any foreign object that is not living tissue) and that takes about 3 weeks to form.
The purpose of the massaging or implant displacement exercises which I commmence usually 1 to 3 days after surgery is to get the implant moving to some degree as the capsule is forming. This in term helps prevent capsular contraction and actually aids in accurate placement of breast implants and also heps prevent "high riding" implants which occur in my opinion more frequently in patients with well developed musculature that do engage in an active massive or implant displacement program. Sometimes massaging is necessary for up to 3 months. Some implants fall fast as soon as 3 weeks, others can take a bit more time and up to 3 months. It is usual for one implant to "drop" and soften before the other.
I hope this helps!