The 'facts' are that these guys get huge in an inordinately short amount of time then claim to be clean...then after filming is finished they seem to shrink to half their size again. If you've ever been in a gym in your life you'll know this is a classic roider pattern.
When you put on the muscle naturally it tends to stay around for longer as your body's natural testosterone levels can support that amount of muscle mass, when you're roiding you're putting on a load of mass that you NEED extra testosterone to maintain. Which is why as soon as they guys cycle off they seem to slim down quite dramatically. Pitt and Cavill are most likely on it for certain roles, The Rock is most definitely on it, Jackman I could maybe understand otherwise as he doesn't seem to slim down between roles and he isn't actually that huge, just very low body fat %. I guess this is all bro science to you though, right?
But yeah I'm an asshole for not taking everything Hollywood guys say at face value when all the evidence suggests otherwise. I guess Travolta also never got a hair transplant, because he's never admitted to it, right? And all those women never got botox or tit jobs? Sure dude.