Toshinori Yagi, aka All Might, was not always a scrawny fellow in My Hero Academia.
Series protagonist Izuku Midoriya is arguably the biggest All Might fan. He always looked up to Japan's top hero.
All Might imbued confidence with his muscular frame. However, Midoriya was in for a shock when he discovered the Pro Hero's secret.
Toshinori Yagi's real form gave him the appearance of a sickly dying man. Before he passed down his One For All (OFA) Quirk, he used it to empower himself.
There is a reasonable explanation for his skinny form in My Hero Academia.
Note: This article reflects the writer's personal views.
How did All Might get skinny in My Hero Academia?
For the longest time, Toshinori Yagi wanted to keep his real form a secret. He wanted to inspire the entire state of Japan.
Of course, specific conditions led to his current state in My Hero Academia.
He was not always like this
All Might Rising was the official spin-off for the My Hero Academia series. It received a manga one-shot and an OVA.
All Might Rising showed the superhero in his early years and depicted him as a very healthy individual. He even had the muscular build that he was commonly known for.
My Hero Academia: Two Heroes also portrayed All Might in this state. It's fairly evident that his skinny form happened to him later in life. Back then, he didn't look anything like his current appearance.
All Might was heavily injured by All For One in their first battle
Sometime in the past, Toshinori Yagi became the first OFA user to defeat the villain All For One. Both were brutally injured in this fight.
In All Might's case, most of his respiratory system was completely destroyed. This would be a historical turning point in My Hero Academia.
The main reason why All Might is very skinny is because of multiple surgeries he has undergone, along with medicinal side effects. His ongoing stress also made his eyes appear rather sunken.
All Might took his Pro Hero job very seriously. Unfortunately, this also limited the usage of his OFA Quirk. He could transform into his muscular form, but only for a brief period.
All Might didn't have much time left, so he needed to find a new successor. This ended up being a Quirkless boy named Izuku Midoriya, also known as Deku.
Sadly, his skinny form is now default
The Hideout Raid ended up being his last hurrah. All Might fought his arch nemeis for the final time. Despite the villainous efforts of All For One, the superhero prevailed by using the last few embers of his OFA Quirk.
Now that it has disappeared completely, he can barely go back to his muscular form. Any further transformation would only last a mere few seconds.
Once his real form was revealed in his final battle, the entire state of Japan was cheering him on. They didn't care what he looked like, only that he could save them one last time.