Board Thread:Theories and Speculation/@comment-27260817-20160131185914

Naruto received chakra from Son Goku prior to deflecting the Bijuu bomb correct? So that should have increased Kurama's power significantly. We don't know how much chakra he gave Naruto, but it's possible that he was able to use it during his tailed beast transformation.

It could also be that Naruto's Bijuu bomb was weaker than the combined one, and that his goal wasn't to match it's power, but to merely send it skyward by positioning it lower than the other Bijuu bomb.

I also think that it was stated that because the tailed beasts were under Tobi's control, they were weaker and were just acting like mindless beasts (I can't remember where this was stated). This may be the reason why they couldn't overpower Naruto's Bijuu bomb.

This is just speculation on my part. I'm not sure if there's an official reason, but it's probably because Naruto's the main character and has plot armor if it had no real explanation. 