Of course Naruto wasn't going at full force during the whole fight. (Naruto could have just barraged Sasuke with a bunch of TBB instead of just one) But as far as that clash a bigger TBB would take more time so that clash was pretty legit.
I should write a huge thread on VOTE2 to stop these bargage threads.
Remember that fight that Taka had with Killer Bee? Remember when Taka was trying to capture Bee but in order to do so they had to fight with an intent to kill?
Same applies to Naruto. Even though he had no intent to kill Sasuke, he couldn't hold back either. Naruto had to use techs with intent to kill to defend Sasuke's killing intent attacks.
Both weren't fighting at 100% because both just finished fighting 2 Juubi Jins and Kaguya
Sasuke needed to use the Bijuu for that extra umph, and Naruto needed Kurama to draw Natural Energy from the planet's surface just to match what Sasuke drew from the other bijuu.
The fight was pretty evenly square give or take.
By that logic, I can bring up a lot of things that even Sasuke didn't do. Naruto was using powers relative to Sasuke's powers. If Naruto could have gone bigger, so could Sasuke. What can be said about Naruto can also be said about Sasuke.
OT: Naruto was using his maximum powers. He was countering Sasuke with equal power to what Sasuke used. Both of them could have gone bigger. But when Sasuke went full power, Naruto also went full power and stalemated showing that he couldn't overpower Sasuke if he wanted. Also, intention to not kill is not correspondent to not being serious.