My approach in Errant was simply this:
Being good at pew pew shooty projected ranged magic doesn't make you weaker, it just doesn't increase your physical abilities. You can still become as physically strong as any ordinary human BUT... you probably don't have lots of the short wavelength magic that can make you extra strong and tough if your magic signature leans blue rather than red.
In other words, what if it's not that Wizards are weak, it's just that wizard = ordinary level of physical strength and normal human limits while Warrior = superhuman physical strength and toughness, and they're fighting monsters who'd kill a normal human easily?