Possible causes i can guess are
1. Could be high sugar/carbohydrate diet making ur gland be swollen or enlarged mimicing jaw fat
2. Genetically maybe you have fat storage at the neck or back of jaw even at low body fats
3. Since you're training masseteers and temporalis muscles the muscles will have microtears and inflammation so there may be fluid retention and pumps
4. It could be stress or bad sleep
To fix them you can read them and fix the issue it's self explanatory but if it's stress or bad sleep fix it if it's 3. It's temporary if it's 2 try to eat high protein and rude sodium levels and if it's 1 fix diet again