All Might real form is very skinny and he was able to do his buff form because of One for All, supposedly. But now he no longer has One for All and he still can do his buff form. Is he supposed to still have some tiny remain of One for All , is his buff form made for comedy purposes or what?
All wrong. All might in his past was always a big buff guy thanks to his training. He never had a "skinny form" his muscle form is how he used to look always before AFO injured him. As all might said to deku, he wasted away from his surgeries and complications. When he uses his quirk his body transforms to its former prime to handle the power. As for why he can still do that form for brief moments may be for comedy purposes or maybe the quirk doesn't fully leave the host after passing it on. If so, hopefully all might will be able to utilize this in a way who knows.