All you have to do is see it. Ichigo says it himself; his power was combined with that of Zangetsu. And as we see, after Ichigo learned to trust Zangetsu when facing his inner Hollow, it was merged and melded with Zangetsu, thus all that power was being given to Ichigo. If you can't understand that then, that's your problem. But really, you're trying to compare Kenpachi from back then to his current self. It's obvious to anyone that Kenpachi has gotten powered up (and frankly Ichigo still shits the bed when Kenpachi wants to have a re-match).