How can I make my jaw slimmer like in the bottom pic? (Photo)

Yes, there are several options to consider for achieving this. Genioplasty, if done correctly and designed specifically to your case, or simply shaving the chin (at the right place), can do that.

The amount of sagging depends on how much reduction is done. However, the approach you show here is really not very good. You end up with irregular bone edges and require a substantial amount of metal to be used. We have been doing this kind of surgery for 20+ years without those problems.

If you are looking for a significant slimming of the mandibular angles, then complete removal of the angles of the mandible is best. Cortical osteotomies will provide much less cosmetic changes. Having performed multiple gender affirmation surgeries, I have not seen sagging or Masseter muscle...

For mandibular reduction, a portion of the mandibular angle is removed with custom guides, and the masseter muscle is not altered. In some cases, I diminish the inner surface of the masseter with cautery but this is kept to a minimum. It is important for the masseter to reattach to the bone for...

It would be impossible to give you a meaningful advice based on these photos. A series of full face photos, profiles, 3/4 views is needed to better understand your problem. In person consultation with a board certified plastic surgeon with expertise in facial rejuvenation surgery would help....

Changing the shape of the jawline is called V-Line surgery. The angles of the mandible and the chin are changed in order to create a slimmer facial appearance. Sometimes the changes are performed by shaving and in some cases, the bone is cut away and plates and screws are placed.