In My Hero Academia, a person's natural Quirk factor may impact the appearance and function of their body in many ways, with prominent examples including Tsuyu Asui's frog-like tongue and Jurota Shishida's ability to become an X-Men-style beast in combat. In rare cases like All Might, though, a person can alter their body with training and sheer willpower instead.
Some My Hero Academia fans had wondered if All Might's brawny muscle form was closely linked to his One For All Quirk or even a separate physical enhancement Quirk in its own right, but the truth has since come out. One For All does not grant a muscle form; it's something All Might accomplished all on his own, and symbolically, it may now be Izuku Midoriya's turn to get a muscle form. This is an essential milestone in his development that may totally change how the world sees him.
The Symbolism of All Might's Muscle Form in MHA
Many aspects of All Might as a character and his One For All Quirk are highly symbolic and thematic in relation to Midoriya and broader shonen staples. Many people in My Hero Academia might think All Might was born an invincible hero, almost like a god in mortal flesh, and that's the image the Pro Hero must maintain to keep society functioning. The truth is completely different, though, and it ties in to both his Quirk and his muscle form.
Unlike characters such as the icy Shoto Todoroki or Momo Yaoyorozu, Toshinori Yagi/All Might was born without talent or even a Quirk of his own. He started from zero -- but only on the outside. What's important is not a person's circumstances of birth, but what they do with themselves and the courage found in their heart. This is why people are chosen to receive One For All rather than inheriting it genetically like most other Quirks.
All Might had what it took to work hard, be brave and become the great hero society needed him to be. Hence, Nana Shimura bestowed One For All onto him, and the air jet hero Gran Torino rigorously trained him to become worthy. Young Toshinori Yagi was reforged as a UA student and a disciple of Gran Torino, transforming from a skinny boy to a musclebound man who could handle the burden of One For All's true power.
Just like One For All itself, All Might's muscle form was something he earned through exhausting hard work and stubborn grit rather than inborn talent. This is fairly similar to how a real-life person can commit themselves to a workout routine to become a musclebound model, except All Might pushes it to a heroic extreme. His muscle form is both the result of and symbol of his hard work as a worthy successor to One For All, so if he can do it, Midoriya probably can too.
How Izuku Midoriya Might Get His Own Muscle Form
It's easy to dismiss All Might's muscle form as an example of "anime physics" with matter appearing and disappearing for the sake of convenience or humor. But in the context of My Hero Academia, which is highly thematic about society and the true nature of heroes, All Might's muscle form isn't a gag, nor is it necessarily unique to him. One person's success in life is evident that others can follow the same steps to achieve the same results, and that applies to Midoriya and All Might as well. Young Toshinori Yagi defied the odds by going from skinny to beefy and gaining One For All, and so far, Midoriya has done much to follow that formula.
Midoriya worked out to become strong enough to gain One For All in a remarkably short time frame, pushing his body to the limit to gain muscle, enhance his durability and more. He has since pushed himself even further to use more of One For All's power, such as 5% or 20% Smash as well as Shoot Style, and no doubt his physical body is keeping pace. Midoriya is a pretty muscular kid, like a junior Olympian, and by his late teens or early 20s, he may be as brawny as his good friend Mirio Togata or even more so.
It's conceivable that once Midoriya reaches a certain point in his physical and mental development, he may gain his own muscle form just like All Might. For years, All Might's own muscle form was living proof of his hard work and heroic spirit, and his protégé is on track to do the same one day. It would unlock the boy's true potential and let him perform 100% Smashes, and he may gain physical speed and stamina to match. All Might is Midoriya's idol, so naturally he wants to emulate him in every way that matters, including a muscle form. In so doing, he would complete his character arc and become the #1 symbol of peace he swore to one day become.