Would masseter Botox get a slimmer look, or buccal fat removal? (Photo)

Thank you for posting your question and photo. When injected into the masseter muscle, Botox can reduce the size of the muscle to provide a defined contour to the jawline and slenderize the lower third portion of the face. The results from Botox injections last for approximately 3-4 months.
For more long-term results, a buccal lipectomy would likely be the best course of action. This procedure removes the buccal fat pad to reduce fullness and refine the appearance of the cheekbones and jawline. Since buccal fat is not regrown even with aging or weight gain, the results of this surgery are considered permanent. I recommend scheduling an in-person consultation with an experienced, board-certified facial plastic surgeon who can help you decide on the best approach.

You should be sen in person to discuss this. From your photo, it looks more like anterior buccal fullness from the buccal fat pad rather than posterior buccal fullness from the masseter, but only a physical examination can make that diagnosis appropriately.

If you look at the back part of your jaw it is already slim relative to the cheeks. There are reference points called bizygomatic width and bigonial width. This is a fancy way of saying distance between the widest part of the cheek bones relative to the widest part of the angle of the jaw. The angle of your jaws is narrow relative to the cheeks, and masseter reduction in any way would create an imbalance. It is a photo so I can't see if your masseters pop out when you clench--if that is the case, botox would help. If you look in the area just under the prominence of your cheeks, you have some fullness, and that is where the buccal fat lives. You also have some jowl accentuation at the jawline and that is what the liposculpting would help with. All told, both would help define your jawline without making it too narrow relative to your cheeks. It's all about balance.

I might offer both options in combination in a surgery under local anesthesia with sedation. Fees in the $8,000 ranges.

Based on the photos it appears that you could benefit from botox injected into the masseter muscle. This should be done conservatively with multiple sequential treatments. You will also benefit significantly from having a buccal fat removal procedure which is performed inside of the mouth thru a tiny incision leaving no visible scar.