I'm not too wise on the subject, but there is a filesize limit on everything (?) in brawl.
To make a "true" one-slot size mod, the animation changes would have to be stored inside the .pac/.pcs. Storing that much animation data in a character's .pac file would likely go over the filesize limit and would cause the game to crash.
I thought the animation data was supposed to be in the FitMotion of the character, and "adding" the YRot bone again with a diferent name and getting the character's texture/vertex pac and adding the same bone there was the only way. ._.
I didn't know the animation data could be saved on FitCharacter.pac