Chewing gum trains your muscles but does not affect thickness
study
Isometric Clenching Exercises can improve strenght and muscle size which is probably what you aim for
study2
theres also an exercise routine called rocabado's 6x6 exercises but this doesnt affect thickness either and had high reports of tmj
study3
a minimal non representative 2subject study showed facial improvement (muscle size) after using jaw exercisers
study4 and another one showed increased cross-sectional area in related facial muscles like the zygomaticus major and digastric
study5
mark as solution