Interesting, but a lot of it, even in people is related to the fetal exposure to testosterone and growth hormone. Short of extreme nutritional issues/disease/etc our size is determined in the womb from my understanding. Another thing is, a lot of it is the right balance of test/estrogen. Men and women need proper ratios, like guys can have too much testosterone and have sexual problems, depressions, etc because the estrogen is too low(pretty sure this is mainly from steroid use). The casque and physical size I'd think would be determined by growth hormones and the hormone exposure from being in the egg. Like a dude can take steroids and workout all he wants, but it won't make him taller/wider etc... granted I think it can make bones more dense.
Feel free anyone to correct me if I got something wrong there
I would think excess free testosterone would make more aggressive males in regards to mating behavior. It has been studied that testosterone impacts the part of the brain that handles risk/reward... more test=more willingness to take risks in order to breed. If that means fighting(humans being a violent species, that makes sense) or running in front of a bird(smaller animals) to impress a female, etc.