Rocks... well, it seem the Giant's rocks don't really follow the normal size progressions. I suspect this is an inconsistency that slipped through the 3.5 cleanup.
Most Large giants have 2d6 rocks, with the exception of the Stone giant who has 2d8. Huge giants seem to use 2d8, even though it should be Large 2d6 -> Huge 3d6 (-> Gargantuan 4d6). The Colossal mountain giant uses 4d8 rocks, which isn't a standard for Colossal weapons. It's closes to 6d6 (Colossal Falchion or Longsword), which would scale down to 4d6 at Gargantuan. (See the "Larger and Smaller Weapon Damage" table, which is much more useful than the "Increased Damage By Size" table.)
Overall, I think 4d6 rocks seem to make most sense. At least it's a standard damage for that size.