I won't lie it doesn't sit right that WB had that little "result" with as much as he was purported by characters to have the strength to destroy the world. We can say it was due to age, or disease but that's just excuses. For whatever reason Oda chose to have WB's damage not look too monstrous despite his description and despite the fact he depicted a Shichibukai who shouldn't even be able to hold a candle to a Yonkou 1v1 breaking an entire island
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So yeah, you can kind of understand what Toshizo meant by underwhelming when one of the weaker shichibukai is able to do that. Realistically WB's level of damage didn't quite live up to the hype but he also barely fought in the war, relative to everyone else.