Does the Breast Implant Pocket Shrink?

The capsule will likely shrink a small amount but there are many different aspects to the breast capsule other than the volume. The diameter and projection of you new implant play a role as does the surface texture.
More often than not some amount of capsule repair is required and is called a capsulorrhaphy. Usually this is done to keep implants from falling to the outside of the pocket.
If this was not performed at the time of your procedrue, you may want to consider wearing a supportive bra while the capsule shrinks down in order to avoid outward movement of the implant during this time period. I often suggest that patients wear these 24 x7 for 6 weeks during the post-operative period. In any event discuss this with your surgeon.

There may be some contraction of the pocket after a smaller implant is placed, however, sometimes it is necessary to place some sutures at the time of surgery to reduce the size. Otherwise, your implants may be felt moving around inside the pocket. Give this some time (a few months) and talk with your surgeon to see if sutures were placed to close the pocket at the time of surgery.

When an initial breast augmentation surgery is performed, a capsule made of scar tissue develops around the implant. This initial capsule may shrink causing a tightness or distortion of the breast called a capsular contracture, which may need to be addressed by your surgeon.
Routinely, after the capsule has formed and stabilized over time, it will not continue to shrink. If a larger implant is replaced with a smaller one, the capsule does not appreciably shrink to conform to the new size of the implant. This will usually result in the pocket being to large for the new implant therefore causing the implant to move within the pocket and migrate laterally. This can be prevented if the surgeon decreases the size of the capsule (performs a capsulorraphy) prior to placing the new smaller implant.
If the capsullororraphy is not performed at the time of the implant exchange, the chances the pocket will shrink enough on its own to accomodate the new smaller implant are not very good.

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.