WARNING: The following contains spoilers for the My Hero Academia manga.
Generations ago, All for One gave his younger brother a Quirk that enabled him to stockpile power. Unbeknownst to All for One, his brother already held the ability to transfer power. These two abilities merged to create One for All, a stockpiling Quirk that can be passed down from person to person. While All for One used his ability to steal and gave away Quirks to tear down society, his brother had a strong sense of justice and wanted to put an end to his tyranny.
Ultimately, he decided to pass on the Quirk so that someone could fulfill the original user's pledge to defeat All for One permanently. It's a fascinating ability that we see wielded by All Might, the eighth holder, and Izuku, the ninth and current holder. Through its long history, One for All has accumulated many secrets.
One for All Wasn't Always a Strong Quirk
Seeing how it's used by All Might and Izuku, it's hard to imagine One for All as anything less than supremely powerful. But at the time of its creation, One for All was actually a fairly weak Quirk, leading the brother to entrust it to future generations. With each new user, it stockpiles more and more power to pose more of a theat.
However, during this process, it's traditionally used in conjunction with another Quirk. For example, Daigoro Banjo, the fifth holder, was born with Blackwhip. Nana Shimura, the seventh holder, had Float. All Might and Izuku have been the only successors to be Quirkless at the time of inheriting One for All. It is unknown how previous holders utilized a weaker version of One for All and built it up to become what it is now.
All Might Didn't Undergo Rigorous Training To Unlock 100%
Before he obtained his Quirk, Izuku had to undergo several months of intense training to physically prepare his body for the strain of One for All. If not, his newfound power would have completely destroyed his body from the inside out. However, even after all that training, when Izuku did receive it, One for All was still barely compatible with his body, and he developed a bad habit of breaking his bones every time he used it. While he's worked on training his body ever since, Izuku has had to learn how to use certain percentages of his Quirk at a time.
This was new territory for his mentor, All Might, who states that he never had this issue and could use One for All at 100 percent upon receiving it. So, how could his body naturally sustain it? While the reason for this is unknown we can theorize that All Might singlehandedly built up One for All to such an extensive amount and it was made dangerously powerful before passing it on to Izuku. Though without knowing how previous holders utilized this Quirk, it's hard to be sure.
All Might's Body Deflation
From his introduction in the series, viewers came to learn that the Symbol of Peace has two forms: muscular and lanky. All Might could only use his Quirk while in his muscular form since his original fight with All for One left him weakened, only being able to maintain that form for a few hours at a time.
When not acting as a hero, he reverts back to his slimmer form and is unable to use One for All. While it does make sense for his extensive injuries to leave him in a state of declining health, it doesn't make sense how All Might can inflate and deflate at will. His ability to deflate is odd and not a side effect of his Quirk. The loss in muscle mass wasn't a gradual thing, as far as we know, and seems unique only to him, so his deflating, uh, quirk, is very unusual for an originally Quirkless man.
Izuku Can Talk to One for All's Previous Users
As One for All passes from one person to another, it also harbors the consciousness of every previous holder, allowing them to co-exist in a subconscious realm. The purpose of One for All is thought to be stockpiling large amounts of raw power to enhance one's own physical abilities, later to be transferred. There is no information regarding this Quirk containing a psychic feature that allows every previous holder to live on inside the Quirk, though.
All Might mentioned that he saw ghosts of the previous holders once in his past and Izuku did so as well while under Shinsou's Brainwash Quirk in the Sports Festival. Later on, Izuku was able to enter that subconscious realm and speak to the previous holders. He cannot yet do this of his own free will, but he's been able to speak to Daigoro Banjo, the Blackwhip user, and All for One's brother. In the current War arc of the manga, the first user is seen warning Izuku of impending doom. These instances suggest that the previous holders can see or sense what is happening in the current time.
One for All Has Multiple Quirks
As well as seeing vestiges of the past, Izuku is slowly becoming able to use every previous holder's given Quirk alongside One for All. This was something All Might was never able to do -- something unique to Izuku, in fact, leading All Might to claim that he will be the one to complete One for All.
Though this begs the question, why Izuku? Why can he see the ghosts of the Quirk's past and why does he have the ability to gain more Quirks? (Other than, you know, being the series protagonist...) Since All Might and Izuku were both Quirkless, he should eventually gain the ability to utilize six additional Quirks on top of One for All. It is unknown why Izuku, the ninth holder, will complete One for All and what that could mean for its continued future after him. However, since his Quirk was created to defeat All for One, if Izuku manages to do just that, then the world would no longer have a need for One for All, theoretically.