@urugiru it hurt because it was a punch with the intent to hurt.
But you can see the entirely different way both are portrayed.
First he stops to think, 'wait, that actually hurt?', then get mad because he took a punch.
Then when it's his time to punch, there is a total different feeling show through the manga, as that is actually something that will be very very bad if she takes it.
Then another boy takes it, and mentions you shouldn't hit girls.
And as you can see, one of them had to go to the infirmary, and it wasn't the one punched by the girl.
Even the fact that one punch was aimed at the gut, and the other at her face was a way to portray the difference in violence.
This really was the mangaka attempt to show the difference between boys and girls.
I'm not gonna argue that the generic male puberty is stronger than the generic female puberty, but those are kids.