Kishimoto always portray them as equals i don't think you're reading the same manga as everyone else even some Naruto fans see the portrayals and actually it's you i should be laughing at for thinking that the 1 tails is stronger than a Susano'o when it have no feats to beat a PS or actual abilities to beat it hell Gamabunta was putting up a fight with it.
1. They were not always portrayed as being equals:
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Unless you're going to say Sasuke had it in him to do the same exact thing. Here's even more proof:
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If they were equal, Sasuke wouldn't have had to display both a superiority and inferiority complex when it came to Naruto. One doesn't have to be a Naruto fan to see that.
2. Shukaku didn't use a Bijuu Dama in that fight and by Madara's own statement, a PS atk is only on par with a bijuu dama. He was obviously saying a bijuu dama from any bijuu > a PS slash, no matter how wide or miniscule the gap is.
''It rivals even the Bijuu'' he didn't say which bijuu and Shukaku is weak Gamabunta held his own against it pretty sure a PS which is far stronger can beat it.
Madara saying "a bijuu" means
any bijuu. Gamabunta couldn't handle taking Wind Bullets from Shukaku. And it's not even Shukaku's most devastating move.
Because the story is about Naruto and Sasuke. The 2 main characters Kakashi and Sakura are the sidekicks.
Sasuke is
a main character. Naruto is
the main character. That's why the story carries his name and his name only. At the end of the day, the story is a lot more about Naruto than Sasuke.