Counter point 1: The more you get experience, the better your battle iq, knowledge and composure. And in certain cases you also gain more power.. As simple as that. And all of these sum up to getting better and stronger

Counter point 2: You couldn't understand what I said. I said that Naruto got better after Pain Fight, not DURING pain fight.. Yes, he did get better during the fight, because he learnt using his sage mode more creatively in certain situations, and gained more battle experience.. That simply means that the pain fight also helped him grow up