I see this a lot with 'linear Fighters' and 'quadratic Wizards'. Could someone explain to me exactly how this works out?
I think the linear vs quadratic refers to what each class is able to do with it's standard action as they advance in level.
The fighter can swing his sword (or ranged attack, or make a combat maneuver). His attack is liner - essentially +1 BAB each level. His damage also increases slowly, probably close to +1/level. (Note: due to items/feats this is probably closer to +2/+2/level.)
The wizard can cast a spell with his standard action, and like the fighter he generally starts with about 3 options in the form of three different 1st level spells he can cast. At each level, he does not get +1 to his spells effectiveness or even +1 additional spells, he gets several more spells, and they are generally much more powerful as he advances. Also, his lower level spells scale up very nicely as he advances as well.
This means that after 20 levels of advancement, the fighter is at +40 to hit, and 1d12+25 while the caster is casting a wish spell, wail of the banshee, stopping time, etc.
If this is a problem, the only real solution is to tone down high level magic. High level casters can easily break the game. Giving those kinds of powers to other classes, just means more classes are capable of breaking the game.