His muscular form is as iconic as his skinny form, but why is All Might getting weaker?
All Might is getting weaker due to a severe injury he suffered 5 years before the start of the story. Surgeries for the injury have wasted away his body, and he increasingly has less time to use his muscular form.
He can only enter his muscular form for 3 hours a day before the pain becomes too much.
All Might explains why he’s getting weaker
In chapter #1, Deku grabbed onto All Might and went for an unexpected fly across the city. The two landed on a rooftop, with Deku using the opportunity to ask if it’s possible for him to become a hero without a quirk. Deku and All Might were able to meet for the first time earlier in the day.
While he explains why he wants to be a hero, All Might’s muscle form begins to shrink and dissolve away. Deku is completely shocked at All Might’s true form. The hero’s body is so skinny and gaunt that his eyes appear to be black as a shadow crosses his face.
All Might pleads with Deku not to share this secret on the internet. He sits down on the ground, lifting his shirt to reveal a gigantic injury to his stomach.
He explains that five years ago, an enemy gave him a grave injury. All Might’s respiratory system was nearly destroyed and his stomach was removed. Due to the effects of the surgeries, All Might’s body has begun to waste away. He can only do his hero work for about three hours a day.
Deku, being an All Might-fanboy, recalls a fight five years ago against Toxic Chainsaw. All Might says that lowlife could never have done that type of damage to him. He intentionally decided for his condition not to be made public, as “a symbol of peace who saves people with a smile, must never be daunted by evil.” All Might says he smiles to stave off the overwhelming pressure and fear he feels as his power slowly fades away.
So that’s the answer. All Might is getting weaker due to a severe injury he suffered 5 years before the start of the story. The effects of the surgeries have wasted his body away and only allow him to enter his muscle form for 3 hours a day.