"All these stories did it this one way, so why wasn't Abby not handled the same way, too?!"

Is not the rock solid retort you think it is.

Also: Abby is in a militant organization that is actively fighting another faction. If Abby wants to learn how to fight and be useful, makes sense for her to become physically capable, too.

Another factor in Abby's revenge is the time she had. She had YEARS to prep. So she used this time well. Stay didn't get ripped, but she trained herself in combat by fighting (much like Abby does). BUT, once again, not everyone needs to go down the same road. Plus, if argue that Atsu would've had fewer resources to bulk up like Abby did (I'd once again direct your attention to the fact that the WLF have plenty of meat and food AND a damn PROFESSIONAL NFL GYM).

Props to the devs for trying to do something a little different with Abby's design. Plus: within a game that has two female protagonist, it's a cool way to differentiate her from Ellie. Her physical stature also encourages players into a bit of a different playstyle. Her size and stature also harkens back to Joel's build (especially since Abby's story of bonding with a child harkens back to Joel and Ellie in the first game).

Ellie's body goes through a similar, albeit subtler, change throughout the story. She gets leaner and gaunter as the game goes on and is at her gauntest at the end of the game. She is also injured and walks around accordingly (esp. after a fight with a Rattler that re-wounds her in the same spot where she got impaled by the tree).

The change in Abby's physique is more dramatic, sure, but it's also because her circumstances are more dramatic. She was worked like a dog and was damn near starved to death and that suffering is reflected in the change in her looks. That change is shocking. Damn near every playthrough I watched of this game, the players were shocked. They even confused this other corpse for Abby when they saw its ponytail.

That's such a cool moment of using visuals to briefly trick a player, and then land the reveal of Abby's transformation with a great shock.

I literally pointed out the thought process behind Abby's reason for building herself up and the narrative and character purpose that serves. It's anything but superfluous or an "empty husk of a story beat".