This "imbalance" partly reflects the old AD&D system that is the foundation for the BG series.
Mages in the AD&D environment were meant to be very, very powerful, but not until late in the game. Fighters and their ilk were also very powerful, but their "power curve" was very sharp at the lower levels, then flattened out. A magic-user's "power curve" is very flat until the middle levels then jumps dramatically at the higher levels.
The idea behind this is that few mages survive the adventure process because of their low HP, high AC and narrow weapon selection, but those that do attain the pinacle of power available to mortals.
Jesus saves! And takes half damage!
If brute force doesn't work, you're not using enough.