What Are you Talking About?
Baku Is Large Enough to Dwarf Sasuke's Susanoo Yes.
But look at the Size Of Madara's Perfect Susanoo
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.
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Madara's Perfect Susanoo is Far Larger than Itachi's and Sasuke's Susanoo. His Perfect Susanoo Extends Into the Sky.
This is Sasuke's Susanoo
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Baku is Much Larger
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Baku is Much Smaller Than The Perfect Susanoo.
Baku is Probably Half of the perfect Susanoo.
Perfect Susanoo is Far Larger than Baku.
And The Nine Tails is Far Larger than Baku.
That claim is just outright wrong. What you, and everybody else, must understand is that there's no constant measurement or relation of size in Naruto, which I'll display based on five examples.
1.
The first one is quite old. During his fight with Pein, Jiraiya was drawn on Gamaken's head for a good amount of time.
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One could think that Gamaken was intended to be a smaller toad, as he looked way smaller than Gamabunta, for instance, compared to Jiraiya or other humen.
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It's apparent that, based on those pages, Gamabunta easily dwarfs Gamaken in terms of size. However, when Bunta and Ken were both summoned to Konoha's ruins, they were equal in size which, based on the pages I've presented, makes absolutely no sense.
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Once again, looking at how big Jiraiya was compared to Gamaken and how big Naruto was compared to Gamaken, reveals that size is no constant factor. Gamaken would've had to suddenly expand twofold for that to make any sense; as long as Jiraiya is not as big as Gamakichi, which he's obviously not.
2.
The next jump in size happens, quite funnily, within a few pages, and relates to Itachi's first usage of Susanoo. On page two of chapter 393, the drawn parts of Susanoo look absolutey gigantic; the lower part of Totska no Tsurugi's sake jar easily dwarfs Itachi, while Yata no Kagami even trumps that.
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On the next page, however.. well, you do the maths.
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That page does, once again, contradict its size a chapter earilier.
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3.
This one goes without saying; just look at Tobi, Kakashi and Gai.
Gyuuki's hand in chapter 567:
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Monstrous, isn't it? Gyuuki's hand in chapter 595:
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Gyuuki's hand once again in chapter 609:
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4.
This one goes, like the previous one, without words. Just take a look at the massive crater Ae smashed into the ground with Raiga Bomb.
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This is how it looks a few pages later.
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5.
Last but not least, it was explicitely stated that Yamata no Jutsu boasts a body even larger than that of Manda. Comparing the two, however, shows the complete opposite, as one of Yamata no Jutsu's heads isn't larger than Orochimaru.
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There are various more examples, but you should get the point. In essence, size isn't always reliable.