Board Thread:Episode Discussions/@comment-1018594-20150604121840/@comment-26275777-20150605234839

Chakra control. Nature Transformation doesn't just take a lot of chakra, it's an advanced form of chakra control. There's a reason why few genin know it and why Naruto didn't start learning it until after the time skip. But apparently five-year-old Obito had good enough control to convert at least part (if not all, since he's not an Uzumaki and there's no indication he had more than average reserves) of the chakra he'd moulded to actual fire. That's at least equivalent to Sasuke at that age, and perhaps even better, because if I recall correctly it wasn't until the year the clan was massacred (when he would've been seven/eight) that Sasuke started learning the Great Fireball.