Surgical reduction of the lower half of the masseter muscles is best done by electrocautery through intraoral incisions as opposed to actually trying to cut out the muscle. By cauterizing the internal surface of the face, some permanent loss of muscle mass results from the thermal injury...
This would need a surgical approach and honestly I feel would make you look less masculine. I suggest a formal facial sculpting evaluation. Best, Dr. Emer.
This is my expertise and a huge portion of my practice. Asymmetry is normal and expected. Buccal fat pad gives slimming not symmetry. Masseter botox will make you slimmer and make the jawline more loose. Fillers help give shaping and contouring. If you want perfect or close to perfect symmetry...