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Realistically, while Tobirama's recent part two Suiton feats are top-notch, in terms of scale this one Jutsu Hiruzen copied outdoes anything Tobirama has displayed in terms of size.
By all logic there is no valid reason for Tobirama to be below Hiruzen in Suiton, if all goes well he'll show off what he's capable soon; i.e, something bigger than this.
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That Suiton is so big the alliance look like little dots on the map.
My point being is that when people ask why Tobirama has so much Suiton hype it's for reasons like this.