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...