-ga will do more dmg to a single then a normal 1 target spell.
The way I look at it (may not be right but makes sense to me), is that for a -ga spell, lets take Firaga and Fire. Fire will target 1 mob and do dmg. Firaga will target as many mobs in its area. The more mobs mean the Firaga dmg, which could be considered to be at 1 dmg limit, would be split among the mobs.
Example:
Fire vs a gob - 100 *making these numbers up*
Firaga vs 10 gobs - (lets say base firaga dmg is 1,000. 1,000 divided by 10 = 100 per gob, so each takes equal dmg).
But Firaga vs 1 gob - 1,000dmg because that base dmg of Firaga has no otheres to split between, so that 1 little gob takes all the dmg.
Hopes it makes some sense
I don't know how they actually calculate the dmg and all, but this is how I look at it
