How was Shoto Todoroki able to resist One for All?
The answer is basically given in chapter 22 of the manga. When Midoria tells All Might that he managed to hit Nomu without breaking his arm, they come to the conclusion that Midoria subconsciously limited his power output to avoid causing lethal harm to other humans. Mostly possible, that during his fight with Todoroki, he has limited his power in the same way.