Yeah, I don't think it is working right. Most of the other incresed abilities are higher (compare minor and major healing). I have also, rarely, had it leave fires in empty squares that would harm foes entering...but again, rare.
So far, I damage the enemy more with the Major Barrage than with Minor. When checking this, make sure that you are fighting the same type and LEVEL of opponent to check for any differences. If you aren't able to see the Opponent's level, I would think that how many Hit Points (Health) they have would be a good indicator.