I'm afraid you have no idea what parrying is. In reality, the smaller gunblade is far superior weapon to parry with than the the Dark Knights greatsword or the Warriors greataxe.
Parrying is an act of deflecting the attack to the side. It does not require a lot of power. It requires speed and precision. That's why SMALL weapons are used for parrying. Ever heard of
parrying dagger?!
Large weapons are better for BLOCKING, but weapons shouldn't be used to block in the first place. That's a last resort maneuver because it can result in the weapon being damaged and made unusable.