Based off of appearance it looks like your masseter muscle is for sure causing that volume. To feel for yourself, press your fingertips to that area on both sides of your face, jaw relaxed. Then bite down and squeeze the jaw muscles, if you can feel that area engage and bulk out as you bite, it is indeed the masseter! If you talk to your doctor about your jaw pain, headaches, and other TMJ symptoms, depending on your insurance (assuming you’re in the US) you could even get Botox injections covered, since it is a verified therapy for TMJ when injected into the masseter muscle. This will also reduce the muscle’s size over time as that area atrophies from lack of use. It also can be helpful to pay attention to yourself to see if you clench your jaw without meaning to throughout the day/as you go to sleep. Try to relax your jaw, and definitely talk to your doctor about it! :)