I think if you take a look at the *colossal* amounts of damage (and usually for less mana; faster casting; and/or incorporating extra effects like stunning or interruption, or maybe negating saves, SR, etc.) or, for instance, the difficulty of saving against just a simple mez (DC 10 + Cha modifier + Int mod + spell level!! -- even without Heighten Spell...) from even a lowish level enchanter, for instance, you'll see that a d4 for hp is not too bad a thing...
Not that divine casters don't get big damage spells, too, but in a straight spell battle, I think the arcane guys definitely have a considerable edge.
Just a couple of quick examples. Compare:
Shaman spell, 13th level,
ice strike: (2d10+4)x10 cold damage; 42 mana; casting time 1 full round; range Medium. Average damage 150 cold/75 with save.
Wizard spell, 13th level,
lure of frost: (4d6)x10 cold damage; 53 mana; casting time 1 action; range Medium; no save; no SR; ignores cold resistance. Average damage 140 cold,
period.
or
Wizard spell, 13th level,
draught of fire: (4d6+1)x10 fire damage; 36 mana; casting time 1 action; range Medium. Average damage 150 fire/75 with save (for less mana, and with a faster casting time).
Now, don't forget, on top of this, that most wizards will have Greater Specialization (evocation), further reducing their casting costs by 10%. [Shamans have Greater Spec, too, but chances are they're not going to have evocation as the chosen school, since taking alteration would be FAR more beneficial.] And wizards have the quicken spell ability, so for some extra mana, they're getting two of these nasty spells off each round when necessary.
And don't even get me started on the effectiveness of enchanters...
