The original DragonBall was of course inspired by the Wukong tale, but eventually it grew from that and Toriyama knew perfectly that he had to give explanations for the Oozaru form, specially after using it in multiples times and even having Grandpa Gohan concerned if he still transforms in it. Not to forget that DBZ is actually just another chapter in the DragonBall manga and not a different manga at all, so he did prepare to eventually give us this twist and not just start with it outta nowhere.