The damage is different because the weapon is different. The giants are using weapons both sized and designed for them. Damage of human weapons shows the same phenomenon. A two handed sword may look very similar to a one handed sword, but do different damage because it is a different weapon. Giant swords are not just large versions of human swords, they are different weapons. Dragon claws are different weapons than canine claws. There is a reason we have something called "natural weapons," even though the object used for attack is part of the monster's body, does not mean it is not a weapon. This is not to say that larger weapons, whether natural or artificial, deal the same damage as smaller weapons. Only that monsters deal damage according to what means of attack are available to them and not according to their HD.
I have also toyed with the notion of using only d6 damage from OD&D, but that requires a different foundation for combat than is normally used for D&D today.
