Dice and Magic Damage: is there a pattern?

[Klaus Teufel]

Is there a reason that one spell might use 2d6, and another 3d4?

[OldTrees1]

The die size varies to add some differentiating between the spells. It is different to make it feel different.
Fireball: d6s
Vitriolic Sphere: d4s
Cone of Cold: d8s

Then the number of dice is determined by how many dice of that size are needed to bring the average damage to where it is expected at that level (after accounting for other effects).


Although there could be more to it:
Of course since the actual die size doesn't matter as long as it is sometimes different, the die size can be changed to adjust what the upcasting scaling is. 2d6 averages 7 damage. 3d4 averages 7.5 damage so they are close enough to start with. However the former scales better if they are on the same +1 die per spell level. This means if a spell has a non dice based effect that scales well, having it be a smaller dice size can help balance the spell.