Board Thread:Naruto Discussions/@comment-25075055-20140417224010/@comment-664010-20140417231713

Well adding Yamato to the clone of the First increased the power of the Zetsu, it could be that Spiral Zetsu gets a power boost from carrying him. If all Zetsu are more or less made to the same "specifications/stats/power level" he would need some sort of boost to create the True Several Thousand Hands, otherwise they all would have used it.