For those who don't know a brief description of the 2 major quirks involved in the main story of My Hero Academia are the quirks One for All (All Might, Izuku) and All for One.
First One for All
All Might explains that One For All is the union of two Quirks: A Quirk that stockpiles power and a Quirk that could be transferred to others. One For All allows the user to access stockpiled power, which grants the user momentary superhuman strength at destructive levels, increased agility, and superhuman speed. Izuku Midoriya is the ninth user of One for All.
Now All for One,
All For One allows the user to steal other people's Quirks and wield these stolen powers as their own. All For One also allows the user to transfer stolen Quirks to other people. The user of All For One can combine several Quirks in order to create devastating techniques which would normally not be possible with the usage of only one Quirk.
In the fight between All might and All for one,
we see the combined effects of multiple quirks used together by All for one. Going by the name that is around 10-12 stolen offensive quirks (not to mention numerous defensive ones as well as ones allowing him to live etc.
How is it that the combined physical prowess of 9 individuals (2 of which we know were quirkless) can be stronger than the combination of atleast a dozen Strong quirks, maybe more?
Possibly Related: What does it mean that "One for all" allows to stockpile power? The accepted answer fails to explain how this stockpiling would outgrow All for one, unless its some sort of exponential increase for One for All vs a linear increase for All for One.