If you say that if it took 1/10's of kyubi's power to summon Gamabunta then kyubi wouldn't be known as the ultimate destructive force then what exactly are you implying? Are you saying that it should take less or take more? It would be silly if it took more than 1/10's (that's a huge chunk, by the way). And it seems the reason they sealed it up was because it couldn't be killed or else they would've done it. So he's got to have waaaaaaaay more chakra than 1/10 of a gamabunta summon, it's just that he's giving Naruto the very maximum his body can handle at the time. I know I know you're going to start bringing in the "He had more chakra when he fought Haku" and you're right, he did. But that was fueled by anger, not immenent death or possible victory. Need to take that into account.

Edit: Isn't it a bit strange that we argue the physics and reasonings behind some artists minds and a cartoon? I'm not saying that we shouldn't do it, but doesn't it just seem a little.....pompous?