I'm aware that Naruto was using his full power. But Naruto was exhausted at the start of the fight and Sasuke had almost all of the Bijuus chakra absorbed and still tied with Naruto. That is not equal.
And Sasuke was not exhausted too? Even though Sasuke had fought less than Naruto, he would still be equally exhausted. Why? Because Sasuke had less chakra reserves than Naruto to start off with. We all know from the beginning that Naruto had a very large chakra reserve, and Sasuke, not so much.
And when Sasuke got all the bijuu's chakra, he had the upperhand. It would be a different case if after he had gained that chakra he got to the same level as Naruto, which he did not. Thus proving that Sasuke was about the same level as Naruto in the first place. It was not until after Sasuke exhausted that resource that he and Naruto were back to equal grounds.
People expect Sasuke to be able to kill Naruto when he gained the bijuu chakra, and when he couldn't, they deemed him weak. However, Sasuke never had experience with controlling bijuu's chakra. So of course he would not use it as efficiently or to its fullest. He didn't even have that much experience with his rinnegan. This should be common sense.
