Heavy lower face. Is this buccal fat or a large masseter? (Photo)

Great question, and thank you for the picture! I agree with the prior sentiment that an in person consultation is the best way to ascertain which would benefit you the most.  That being said, here are a couple of points that may help clear things up.  As far as buccal fat is concerned, its great when you feel as though you have chipmunk cheeks.  When patients have high cheekbones, after doing buccal fat reduction they get improved definition of the cheekbones and contour by creating a shadow below the cheek and above the jawline.  An easy way to see how you would look is to gently, very gently, suck your cheeks in such that they just touch the teeth.  In regards to masseter hypertrophy, when you clench your teeth shut, you can feel how strong and large the muscle is along the angle of the mandible.  This can be relaxed with botox.  I hope some of this helps! Taking a look at your face as a whole is really the best thing in a case such as yours such that we can look at all of the right proportions, etc.  Best of luck.  

Great question!  I feel that you may have a component of both prominent masseter and buccal fat fullness.  You also seem to have some fullness of the lower, front aspect of the jaw line that may respond well to liposuction.  I recommend an in person exam to give you the most accurate assesment of your options.  It is important that you seek the advice of a surgeon who is certified by the American Board of Plastic Surgery and who has extensive experience with buccal fat reduction surgery.  
I wish you the best! 
Dr. Gabbay.

Heavingess of the lower cheeks could be secondary by buccal fat or enlarged masters. 
ANTERIOR lower cheek fullness is secondary to buccal fat
POSTERIOR lower cheek fullness is secondary to enlarged masseter muscle in most cases. 

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. Facial liposuction can also be used to slim the face. You are a patient that would need and in person consultation for a complete evaluation to discuss your desired results.
I hope this helps