Board Thread:Naruto Discussions/@comment-4366149-20141202054544/@comment-5407586-20141202210141

TricksterKing wrote: ^ No, it's not a percentage thing, it's the amount of overall chakra, the percentage remains more or less the same.

Percentage only comes into play when combining chakra natures with other chakra. The problem is that, the more chakra one puts into it, the bigger the Rasengan gets. The Rasenshuriken's sphere isn't bigger than the Rasengan's (Wind Release one of course).