The other thing you have to think about it that Naruto has been raising his own chakra levels to match that of the nine tail inside of him that way he has more control over the fox and he doesn't have to use it, especially after harming Sakura, so in Shippuden not only does he have the massive power of the nine tail but his own levels are starting to match the nine tails. They gave him a plausible reason to be uber powerful.