I'm only following the anime and I dont know if the manga or some guide says something about this, I dont think there is any hint in the anime about it. IIRC, it was said that Gran Torino started to train All Might in combat right away because he could handle the power of One for All at once. Midoriya in the other hand, everytime he uses One for All 100% breaks his body and to keep from harming himself he has to use One for All 5% or 20% later for what I've read in a wikia. Is it known why All Might could handle the power of One for All at once and Midoriya couldnt?
Sadly there is no explanation that will satisfy you or the question. However I can give a small spoiler about it.
In later chapters it's revealed that All Might once used support items to increase his power output like %10.
That's all we know.