I'm pretty sure a lot of people will think that they already know the reason why One For All became so powerful when All Might and Deku got it. But I'm pretty sure that some do not, but your free to tell me your reason why One For All became so strong over time. However I one reason why OFA became so powerful over time and that reason is accumulated battle experience.
The reason why[]
The reason why I believe that OFA became so powerful is because of the stockpiling power Quirk that Yoichi Shigaraki received from his brother. Because the stockpiling quirk stocks up power I also believe that it also stocks up experiences. A prime example I can give is Avatar the Last Airbender and the Legend of Korra as I believe that OFA is accumulating battle experience from the previous users of OFA just like the Avatar State from ATLA and TLOK. And another example I can use is from Naruto/Boruto with the uses of Shadow Clones and KÄma as both have something to do with accumulating experiences. As Naruto used the Shadow Clones to speed up his training to learn Wind Style, as with the KÄma it also has something to do with accumulating over a millennium amount of battle experience.
How OFA accumulating battle experience[]
I believe the reason why OFA is accumulating battle experience is because it has something to do with all the previous users. As each user hold the power I believe that not only is their original quirk being stored and accumulating in the core of OFA, I also believed that all their battle experiences are also being stored and accumulated in the core of OFA do to the stockpiling of OFA. Piling on any experiences form the pervious users and then piling and added their experiences onto the current user.
How much battle experience did OFA accumulated[]
I'm not sure how much time has passed since Yoichi started to pass his quirk onto future successors. But I believe that each user must have some amount of time gain amount of battle experience. I believe that it had over 100 years of battle experiences as AFO is over 100 years old meaning that it had a long time to gain a lot of battle experiences and that all the pervious user must of had a lot time gain a lot of experiences to store in OFA. Although I don't know how much exactly but I estimated that all the pervious user had obtain up it to be above 10 years or less to gain some battle experience except for Hikage Shinomori and All Might as they both had the quirk for a long time. So for the second user, third user, Daigoro, En, and Nana has to at least have 5-10 years of battle experience as I assume that they must of have a certain amount of time to master the quirk and have to battle against AFO. But because of Hikage decided to focus training the power when he was 22 years old meaning he accumulated 18 years of battle experiences that was later stored up in the core of OFA. I also believe that since All Might had it the longest as he had it for 40 years meaning that he has over 40 years worth of battle experience under his belt. Most likely making OFA more stronger than it was with any of the pervious users. Meaning that OFA has accumulated over 50-70 years of battle experience for All Might. And since All Might had it the longest and then passed it onto Deku meaning he now has up to 50- or over a 100 years of battle experience that OFA accumulated. Making OFA at it's strongest state.
Why I believe that each user is experiencing the experience of pervious users[]
Because OFA stores the vestige of pervious users in the core of OFA meaning that all of the experiences of the pervious users of OFA are flowing through the body of the current user making them a powerful fighter. So this is why I believe that any experience of battles that the pervious users have experienced it will give them their experiences to the current user. And then the current user's experiences will be add to the next users alone with the other experiences. This is why I believe that when All Might had it he must of had over 70 years of battle experiences that was stored in the core of OFA plus his own when he was studying aboard. Which I believe is why he was able to defeat AFO five years before the beginning of the story and five years after their fight. Because so much battle experience was stored in OFA, I believe that it gave Deku most likely 100 years of battle experiences from the pervious users of OFA which is why it was so powerful it broke his bones.
Conclusion[]
I believe that this theory could be true as it explains why OFA is so powerful as it much must of have over a century to store many years of battle experiences to make it so powerful that it reached the peak that it would break Deku's bones when he's not using at the level he can handle at.