Just look at Tito's head and jaw structure in his pre fight interview at UFC 12 back in 1997 and compare it to now. Yes, he has used steroids/HGH. Sorry but your jaw and head don't experience a massive growth spurt on their own.
They do when you have jaw problems and tension heaches. I've had TMJ (temporomandibular joint disorder) for the past 4 years and I can tell you for a fact that my head and my jaw's gotten a lot bigger. A lot bigger! And I've never even touched steroids in my live, nor took any supplements or any other kind of performance enhancing pill or drug.
I'm telling you man, my jaw and head has gotten a lot bigger. When my jaw pain and tension headaches get better, my head and jaw (the muscles as well as the bones) get smaller.