Board Thread:Versus Debates/@comment-30426414-20161228222316/@comment-25122992-20161229165029

Well, Itachi was going full-out with his chakra, he didn't spare hiself at all, since he knew it was the battle he's going to die in, and since we know that some amount of chakra is necessary to keep human alive in the Naruto universe, we can be completely sure that Itachi spent this amount as well. Yeah, Itachi used his MS techniques, but any of them was used only once + fireball and Sharingan genjutsu, while Sasuke was almost spamming his fireballs and chidori (though Chidori was the most taxing technique for him in part 1) + the same genjutsu, weapon summoning and high-level fire dragons. And Oro Substitution took a toll on his chakra too. So it's normal that they ran out of chakra almost simultaneously. Moreover, it was dangerous for Sasuke to spend all his chakra, since some of it suppressed Oro in his body, so after his Kirin usage, he still had some amount left, though he lost it attempting to get an additional power from CS to fight Susanoo Itachi.

As Tobi said, Sasuke was spamming his Susanoo too much after its awakening, while he continued to use other MS techniques as well (Amaterasu and Kagutsuchi). It explains everything. While Itachi was getting blind from MS throughout the 5-7 years, Sasuke reached this point in mere weeks or at least months. In his last battle, Itachi had nothing to fear while using Susanoo, he knew he was going to die anyway.