I believe as of 3.5 the damage should be 3d8, one increase from 1d10 to 2d8 and another from 2d8 to 3d8.
From SRD
Table: Increased Damage By Size
Old Damage (Each)* New Damage
1d2 1d3
1d3 1d4
1d4 1d6
1d6 1d8
1d8 2d6
1d10 2d8
2d6 3d6
2d8 3d8
* Repeat the adjustment if the creature moves up more than one size category.