As weapons get larger or smaller, they tend to deal different damage dice.
Most weapons list damage for Small and Medium weapons: http://www.d20pfsrd.com/equipment---final/weapons
There is also a chart for Tiny and Large weapons: http://www.d20pfsrd.com/equipment---final/weapons#table-tiny-and-large-weapon-damage
But what about Fine or Huge (and bigger) weapons? Normally I would try to extrapolate some formula from the existing tables, but those tables seems completely ad-hoc and a little random. Notice how both a 1d12 and a 2d6 become 3d6 weapons at large?
In my specific case, I have a character with the Expansion power and he's going to be a Huge creature wielding something that does 2d6 at Medium. That's 3d6 at Large, but what is it in Huge? 3d8? 4d6? 4d8?
An official reference would be nice, but even just some stats from a known Huge creature wielding a similar weapon.
Bonus Question: Is it just me or are the damage dice really non-sensical? Weapons double or halve in weight as they change size, shouldn't the damage dice be doing something similar? I mean if a 10lb weapon deals 1d10, shouldn't a 20lb version of the same thing deal more than just 1d12?
EDIT: Looks like 3.5 has a full table for this. The general formula is +50% rounded down. However, the rounding down is quite significant at larger sizes.