One of my masseter muscles are bigger than the other, how can I fix this? (Photo)

Botox combined with radiofrequency can slim the masseter and improve symmetry. I recommend getting a formal evaluation with a facial sculpting expert. Best, Dr. Emer

Botox injections in the masseter muscle are the way to go. More units can be placed on the side that looks seemingly more hypertrophied. Multiple sessions may be needed to attain perfect symmetry. In the interim, you can also opt for radiofrequency-assisted facial contouring for reshaping lower face.

There are a variety of causes to lower face asymmetry and impossible to tell from one photo. As others have mentioned everyone has some form of facial asymmetry and is completely normal. The underlying muscle may be asymmetric, if that is the case it would benefit from Botox treatment. There may be asymmetry of the underlying lower jaw, this can be corrected with re-contouring or jaw surgery, fillers may be an option to hide asymmetry as well. Although a clinical exam by someone who images the bone and can evaluate the soft tissue is the only way to know what options are right for yo. 

The amount of asymmetry that your face shows is actually quite normal in almost every single individual. If this however bothers you so much so, I would recommend that you visit a plastic surgeon like myself who will determine if Botox will be of benefit. The other option may be trying to create the smaller size looking like the wider side of your face which is the right side in your photograph. Fillers may be used in this case. Good luck from New York City