What can be done about facial asymmetry / uneven masseter? (photo)

Even if somebody only chews on one side, since the mandible is a solid bone connected at the TMJs on both sides, the master muscles tend to get larger relatively symmetrically. Perhaps you have an underlying bony asymmetry. An examination would be necessary to answer your question. I hope this helps. Sincerely, Dr Joseph

Thank you for posting your question here!My answer will be based on the limited informations and photos provided, for a more detailed and personalized plan I would recommend consulting with a plastic surgeon in person.
There are multiple reasons for having facial or masseter asymmetry , including favoring one side vs the other while chewing , grinding your teeth etc.As for improving the symmetry , botox is one way of doing it , it will take a few months for the bigger masseter to atrophy for you to notice the improvement . other options involve volumizing the opposite side with fillers etc , this will probably give you more immediate results , it wound make the muscles more symmetrical per say but on the outside your lower face will look more symmetrical.I hope this helps, best of luck!