Silicone Chin Implant Absorbed by Bone?

You are right, there may be changes happening to the chin even after placement of a permanent silastic implant. your chin is probably not where it would be without the implant but there are 4 factors to consider:
- bone resorption (usually 1 to 2 mm, but it may be more)
- normal reduction of bone thickness
- normal reduction of volume of the overlying soft tissue chin pad
- droopiness of the chin soft tissues.
Placement of a larger size implant is a relatively straight forward option. Alternatively, injectable fillers or fat grafting may be helpful. Of course, a careful examination comes first!

You are correct bone resorption can occur with a chin implant,
The exact reasons for this are not known,
Newer implants have been designed that are either fixed to the bone or have a porous structure that promotes tissue ingrowth and is claimed to minimize bone resorption.
Going with a larger implant may provide cosmetic improvements but it may lead to further bone resoprption.

The mere presence of your chin implant is not a reason to remove it, unless it's causing you problems. There is no risk of leaving it in place if it's not bothering you.
If you're unhappy with your appearance, you should consult your surgeon. If that's not possible, you should see a board-certified facial plastic surgeon, experienced with chin implant surgery for an in-depth consultation.
I hope this helps, and best regards.