I been trying to figure out how come the previous wielder of One for All did not know about these Vestiges OR the Quirks that come with them. This is a Shonen Manga which means the obvious answer can be that the Vestiges showed themselves to Deku because he is the "chosen one" typical plot device.
However, that is boring and I have another idea. Although I see merit in what Caliburn said about One for All being a reinforcement and defaulting to physical enhancements for Quirk-Less individuals, I cannot see that being true. All Might would have understood at least that much considering he inherited the Quirk from someone else. Unless she too was Quirk-Less, the type of information should have been known.
Considering that All Might explained the Quirk as simply physical enhancement to Deku, its safe to assume that One for All as a Quirk is just Physical Enhancement when in use. I am thinking the matter might be that All Might subconsciously triggered 1 of the 6 Quirks that Deku is going to manifest. All Might has a Muscle Form that is odd because he is supposed to be Quirk-Less AND Quirk-Less people just do not have extraordinary abilities like changing muscle mass.
One can contribute his injury he suffered to be the cause of that form, but for me that just doesn't make sense. If that was revealed to be the case, I would view it to be poor writing honestly. I am thinking, since All Might shown a natural ease to mastering One for All, that he went even beyond and simply mastered another Quirk. If I am correct, we could be seeing Muscle Form Deku.