Actually naruto mastered kyuubi and sage mode in a months time u can say i mean how long could it have been since the pain invasion ok mate 2 months in manga time.
Its not really about the learning stuff and it also seems obito was taller than kakashi was during the gaiden arc i re read it last night and kakashi wa also pretty tall during the kyuubi attack so who knows..
Its just that obito was how do u say it......
Fucking crushed by a buncb of fuckkng boulders there is no saving such a thing. But then again this is a childrens manga so really how many 10-12 year olds actually pay attention like us lol... I know its no excuse for kishi but hey it is what it is.