OK, so
Half Giant has powerful build.
Lets take heavy crossbow as an example. A medium heavy crossbow does 1d10 damage.
Powerful build: A half-giant can use weapons designed for a creature one size larger without penalty.
This means I can use a large heavy crossbow and it will deal 2d8 damage. (from the weapon damage size chart 1d10 medium is 2d8 large)
Let's say I cast Gravity Bow(Any arrow fired from a bow or crossbow you are carrying when the spell is cast deals damage as if one size larger than it actually is. )
These means that the large heavy crossbow counts deals damage as if it were huge. Looking at Improved Natural Attack for extrapolation, because there are no rules for using a huge heavy crossbow that I can find 2d8 would become 3d8. http://www.d20pfsrd.com/feats/monster-feats/improved-natural-attack
so we have a medium half-giant wielding a large heavy cross bow, that deals damage as if it were huge. For a total of 3d8 damage.
Now let's say I use the power expansion(before casting gravity bow) to grow to large. I know, we are getting crazy here.
Expansion: All your equipment, worn or carried, is similarly expanded by this power. Melee and projectile weapons deal more damage. (projectiles deal damage based on the size of the weapon that fired them)
Ok, so now we have a large half-giant wielding a huge heavy cross bow, that deals damage as if it were gargantuan. For a total of 4d8 damage (once again based on improved natural attack).
Now, expansion is interesting, because it can be used to increase more than 1 size category. Let's say I used it again to become huge instead of large.
Now we have a Huge half-giant wielding a Gargantuan heavy cross bow, that deals damage as if it were Colossal. For a total of 6d8 damage (once again based on improved natural attack).
Now this seems like a reasonable interpretation of the rules. (Since it really isn't covered).
Let's go off the reservation now, let's say expansion could increase your size by 3 categories. What would happen?
Would it be a Gargantuan half-giant wielding a Colossal heavy cross bow, that deals damage as if it were Unknown(1 size larger than Colossal)? For a total of 8d8 damage (once again based on improved natural attack).
Then lastly if Expansion could grow you 4 categories. What would happen?
Would it be a Colossal half-giant wielding an Unknown(1 size larger than Colossal) heavy cross bow, that deals damage as if it were Unknown(2 sizes larger than Colossal)? For a total of 12d8 damage (once again based on improved natural attack).