Androgens are the primary driver for mandibular development during puberty.
So basically: Higher testosterone -> bigger lower third.
Why it works
Don't listen to any copes in the internet, hormones are signals that tell tissues basically what they have to do. Your bones are tissues too, the more signals your bones receive the better they will develop including your mandible.
What to do
It is pretty simple. Try to maximize your testosterone levels during puberty. Obviously don't play around with your hormones but try to get a testosterone replacement therapy prescribed by a good doctor to be supervised.
Natural "testmaxxing" remains cope and if you persist to believe in it you are a retard. The only thing that works is staying healthy, you won't reach 1000+ ng/dl naturally if you don't have the right genetics. That's where exogenous hormones start to get interesting.