The Plain Salary Man Turned Out to Be a Hero: He Was Invincible, Only to Be Exposed on His Niece's Dungeon Stream - Ch. 23

[MangaDex]

 

[brndndss]

MC under so much stress, whoever this muscular challenger is just gonna crushed like a bug

 

[welcome2atlantis]

So when under stress, MC's body grows larger and muscular.
If I had this trait, I'd be Hulk.