One of my masseter muscles are bigger than the other?

I would get an examination by your doctor to make sure that it is indeed the masseter muscle that is enlarged. The parotid gland is in that area as well, and it may be that it is enlarged due to some other process.  A routine head and neck exam can confirm that it is the masseter.  If it is indeed just masseter hypertrophy, Botox can address this or you can chew more on the other side to strengthen the side that is smaller.

Botox works very consistently and well in this area and it is a very effective way of improving (notice I didn't say curing!) the asymmetry. Of course, it is important to be evaluated by someone with experience in both operating and injecting in this area because other anatomical variants can also demonstrate themselves in this manner. Once this is confirmed to be the case, however, a moderate amount of botox injected into the LARGER or bulkier muscle is very helpful to treat it. The botox may wear off around 3-4 months, so will likely need reinjection a few times a year to maintain results.Alternatively, if you would rather increase the size of the smaller muscle, there are a few options. One is to have fillers injected deeply onto the bone to help volumize the smaller side. Some patients choose to have Sculptra injected (which works by stimulating collagen over time) and others choose fillers like radiesse or juvederm to achieve this result within a shorter period. There are no FDA-approved injectables for this area, so it's important to use someone with plenty of experience.Finally, there are jaw implants that are permanent and can be placed through incisions in the mouth to help improve symmetry on the smaller side. Most patients prefer to use injections initially, and if they are happy with the symmetry, and need a more permanent solution, will have an implant placed.Best of luck in your quest!