Buccal fat or masseter muscle? (Photo)

It is likely to be the buccal fat, based on the pictures.  The best way to tell would be with a physical exam, obviously.  That being said, there are some maneuvers that you can perform at home to help you answer your question.  the way to isolate the masseter muscle is to clamp down and clench your teeth together real tight.  This will flex your masseter muscle. As you feel along the angle of the jaw (towards the back of your jawline) you will feel the muscle belly.  You will also be able to feel the border of the muscle.  In front of the muscle lies the buccinator muscle and the buccal fat pad.  By doing this you should be able to differentiate yourself between the two.  Treatments vary - buccal fat excision/reduction works best (i like the intraoral approach), vs for the masseter (botox is a great starting point).  I hope some of this helps! Best of luck. 

You are a good candidate for 
BFPR (Buccal Fat Pad Removal)
When a patient has a full and round face and despite weight loss the
fullness persist or if the patient is in his normal weight and the
chipmunk faces has stayed all the time, a bichectomy or buccal fat pad
removal is useful. It´s true that with aging the buccal fat pad tends to
diminish in volume do to atrophy, but if the volume is excessive a
bichectomy will not hollow the cheek overtime. Many patients are happy
with the results, a proper selection is a very important manner. Usually
the procedure takes less than 10 min and you can go homethe same day.
You'll be able to go back to work on the 3rd day post-op but 1 week off
would be more comfortable.

Asymmetric face could be secondary to differences in the fullness of the cheek bones, buccal fat or masseter muscle in one side of the face compared with the contralateral side. Depending on the cause of the asymmetry, one or more procedures could be performed. Asymmetric buccal fat removal  (either removing more buccal fat from one side or removing fat from only one side) could help to achieve more symmetry of the lower ANTERIOR face.Asymmetric Botox treatment of the masseter   (to make the muscle shrink) could help to achieve more symmetry of the lower POSTERIOR face.Report
Patients interested in asymmetric face improvement/balancing  should make a consultation with a board certified plastic surgeon specialist in buccal fat remova and other procedures of the face.
John Mesa MD
New York City and New Jersey Buccal Fat Removal Specialist

Buccal fat excision is an appropriate surgical procedure for patients that of overly full and wide cheek areas. In common terms, it is good for patients who have chipmunk cheeks. Liposuction can be used to further slim the face. In reviewing your pictures, is difficult to tell if you're a good candidate because your cheek area is slightly full but not overly full. You are a patient that would need and in person consultation for a complete evaluation and explanation of the positives and negatives of this procedure in your situation.
I hope this helps.