I go into this in alot more depth in a thread I made a while back which you can find {
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But the short answer is after the pain arc you begin to see large progressions in power that broke the proverbial 4th wall. Throughout the series Kage level was as strong as someone could get and everyone was compared to it, "Ok Zabuza is good but I dont think he is Kage Level, ok Sasori is good but I dont think he is Kage Level, ok Diedara is good but I dont think he is Kage level."
But towards the end Kage level lost all meaning, We see Pain destroying entire villages single handedly, Madara beating 5 Kages and the entire shinobi alliance without breaking a sweat, the Juubi, etc... Out of nowhere we had these large unwarranted progressions in power that we didnt understand.
I go into this in alot more depth in the thread if you want to read it.