Stone giant bash doing tiny dmg

What level were you at when you did this? Right about L20?
Generally speaking, bash damage is 20-30 for a stone giant at this level.

Wounds: 17-20 damage
Mauls: 21-24 damage

This looks right, given assumptions.
You're also not going to do the double damage every time, much in the same way that a fire giant's bash isn't fire damage every time.

This is a very small sample size. Optimally, I'd take all the bashes you can for an entire level.
Look for jumps in damage.
For example, a L34 stone giant should be in the range of 27-51 (avg 39), decimate to lacerate (average mutilate).
If you see anything from dismember to demolish, there's your double damage.

We'll call this closed then.