Board Thread:Naruto Discussions/@comment-26479810-20180620141830/@comment-5311048-20180620163159

Well, I'd think it had to do with the time that passed that he was practicing sage mode. And/or maybe he just didn't eat all that much during his training just to be able to fight his mother. There is no actual evidence or clue, but I doubt it was sage chakra that did that.