Hmm, let's see. He goes from using a V4 Susanoo to using a Perfect Susanoo the size of Naruto's Avatar. That requires a boost in chakra reserve and chakra potency. Besides, not sure how you can make a claim like that when you have no idea how much of Itachi's power that Sasuke got. But considering the resulting power from their little fusion is enough for him to note it how he did, I'd say that it isn't insignificant like you are trying to make it seem.
But then again him manifesting PS is proof enough that the boost is not small.
And the only different thing happening is that they aren't fusing their ocular powers with anyone elses. Sasuke is, that's why he keeps his original abilities, but gets them enhanced.