genetics. period. whether you're natural or juiced, genetics are going to be the ultimate factor.
think about it, some guys can half-ass it in the gym and eat poorly and still look built as hell, some guys lift hard 4 days a week and diet perfectly but struggle for each pound of muscle. some guys can do a 250mg/week cycle of test and gain 45 pounds, some guys do 750mg/week and just turn red and bloated.
genetics are the bottom line.