All Might has been Izuku Midoriya's role model in My Hero Academia ever since he was a little boy, from the moment he first saw him saving lives with a smile on TV to long after he adopted his superhero alter ego, Deku. It's not just because All Might is the symbol of peace, though this is a major contributing factor to Deku's rampant fandom. It's because of All Might's incredible strength and natural abilities. How could Deku possibly surpass a hero who's held onto the number-one spot for years even though All Might thought Deku was worthy to inherit One For all?
Despite All Might being born Quirkless like Deku, All Might possesses all of the natural qualities of a hero. In fact, All Might has confessed that he's always been able to harness 100 percent of One For All's power without experiencing any side effects while Deku can't do so without badly damaging himself. In the Anime, Deku can only safely use 20 percent of One For All and, for long-range attacks, only unlock 20 percent momentarily. But the manga just revealed that Deku is destined to surpass not just All Might but all of his predecessors.
In chapter 212, Deku loses control of his powers during a competition between Class 1-A and Class 1-B, but the power that overwhelms him isn't One For All. It's a new type of energy that he can't stop pouring out of himself. When Hitoshi Shinso uses his Brainwashing Quirk to keep Deku under control, Deku finds himself in that strange world where he can not only see but interact with the One For All vestiges.
This time, one of the predecessors speaks directly to Deku in chapter 213, revealing that the power that Deku lost control over was his Blackwhip Quirk. He lent his power to Deku because he felt it would help him out in the competition. Not only will Deku get to harness Blackwhip, which is much more powerful than it was back in the day, but he will be able to wield all six of his predecessors' Quirks. While this alone is incredible, the vestige then says that the core of One For All has been growing in power ever since its creation and that Deku just happens to be the one who will complete One For All.
While it's unclear what completing One For All entails and how it will manifest in My Hero Academia, it's sure to be more powerful than what All Might could channel during his heyday. Then there's the fact that Deku will soon have control of six Quirks and receive guidance from multiple predecessors while All Might could only ever wield One For All and just had one hero to guide him. So even though Deku can only currently wield 20 percent of One For All's power, it's quite clear that he will surpass All Might once he's able to unlock its full potential.