Procedure to reduce platysma muscle size? I have a recessed chin, and under my chin is some sort of large muscle. (Photo)

It does appear that you have a recessed chin and some excess fatty tissue under your chin/ jaw. A chin implant will help improve your profile by projecting your chin and it will also stretch the tissue under your chin as well creating a small amount of improvement in your neck. The best result will require removal of fat from under the chin as well and this can be performed with liposuction. In some cases, deeper fat under the muscle, must also be removed and the muscle sutured back together. This will usually create the profile that you are looking for. I hope this information is helpful for you.Stephen Weber MD, FACSDenver Facial Plastic Surgeon

  The photographs demonstrate a small amount of fatty deposits in the neck, a short plasma, and a weak and recessive chin profile. A chin implant would  give you the most benefit to give added projection to the chin area. This will only give a mild improvement to the neck. If you're wanting a significantly angulated jawline, then a platysma-plasty with sutures applied to the submental area is required. For many examples of this combination, please see the link a below

The contour in that area does involve several features.The main issue however is the distance from the throat area to the chin point. If that distance increases then the muscle will flatten somewhat under the chin with a sliding genioplasty which tensions the muscle as opposed to an implant. A second issue is fatty deposits but you are aware already of that part. The manoeuvre you do does raise the floor of the mouth which in reality is just above the skin and fat areas