Does the Breast Implant Pocket Shrink?

The pocket is make by the capsule with is a fibrous structure.  When larger implants are replaced by smaller ones the capsule size stays the same.  So the surgeon must plicate or suture the pocket to itself to make it smaller.  This way the new implant won't move around in the larger pocket.

Whenever an implant is completely removed or replaced with a smaller implant there is less volume. The skin, breast tissue and the capsule cavity(or space for the implant) may shrink down to the new size. Sometimes only one of these components wil decrease in size, sometimes non of the complonents will shrink and sometimes they all shrink.
I would advise waiting 6 months to see what happens to each of the components and then determine if anything additional needs to be done.
Very often when there is a size change downward or complete removal of the implant surgical reduction of the stretched-out tissue will be done at the same time. If an implant size has been decreased it can also be pleced at a different level, not exactly in the same space as the old larger implant. Under those circumstances you don't have to worry about the old site because the new implant is no longer in that space.