From this picture, do you think I have overdeveloped masseter muscles or a lot of buccal fat? (Photo)

Fullness of the Lowwer cheeks could be caused by either buccal fat, enlarged masseter muscles or both.
Fullness of the ANTERIOR lower cheeks is caused by buccal fat.
fullness of the POSTERIOR lower cheeks is caused by enlarged masseter muscles.
Therefore buccal fat removal and botox treatment of the masseter treat the fullness of the anterior and posterior lower cheek respectively. 

If the fullness is more in the lower anterior cheeks, it may be buccal fat. If it is more in the lower posteriror cheeks, it may be overdeveloped masseter muscles. Or it can be both at the same time. And both conditions can be treated with related procedures. It will be best to consult a board certified facial plastic surgeon in person and discuss your goals and options after a physical examination.

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. 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 an in person consultation for a complete evaluation and explanation of the positives and negatives of this procedure in your situation. I hope this helps.

From your photos I would assess you likely have an combination of anatomical areas leading to what you would like to change. This is the most common case btw. Buccal fat removal and manipulation can be a great and transformational procedure in the right patient.  Often times fat transfer in conjunction with this procedure is able to give a more “ideal” heart shaped face. Also, as you mentioned, overly strengthened masseter muscles can contribute to a widened lower third of the face. To treat the masseter hypertrophy we use botox. That can take some time though to reach effect.  It may seem counterintuitive to add fat when you say your problem is chubby cheeks but what we do is add fat high in the cheeks to the side of the highest and most projected point of the cheek bone and taper it to the lower face. This helps to camouflage overly chubby cheeks.  This is especially helpful in thin women and in combination with buccal fat pad removal is a powerful maneuver. All of this can be done in clinic with some calming medications and local anesthesia.
I recommend getting a consult with a surgeon who does this procedure frequently to get a full facial analysis to make sure your specific goals are addressed.
Hope this helps.
Best of luck.
DrC

Thanks for your question and Pictures
The buccal fat pad is a rounded mass of fat in the middle of your cheek. It’s located between facial muscles, in the hollow area beneath your cheekbone. The size of your buccal fat pads affects your face shape. Everyone has buccal fat pads. However, the size of buccal fat pads can vary greatly. If you have larger buccal fat pads, you might feel like your face is too round or full. You can be a candidate for a Buccal Pad removal surgery.
Best of Luck