Size does not increase the Death Throes damages, only the saving throw.
A 32 HD Frost Worm would look like this:
Gargantuan Magical Beast
32 HD
HP 400 (32d10+224)
Init: +4
Speed: 30', Burrow 10'
AC: 20, FF 20, Touch 6
BA: 32, Grapple 56
Attack: Bite +41 - 4d8+18
Space/Reach: 20'/15'...