I mean, what if I wanted to play a lawful good mage? Should I automatically become more powerful and get some bonus spells because I'm lawful good?
They are at a disadvantage, they can "only" be Lawful good, a limitation the Mage doesn't share. So your argument dies right there. To quote Volourn... you lose.
If you're going to play the ubermunchkin powerhour, it doesn't matter what restrictions are there since you'll ignore them anyway. But the Paladin's "code" is a limitations since there are a lot of things that character can.. and can't do.
If you follow the whole alignment thing.
According to you, anything that's lawful good would get some sort of penalty, so wouldn't it be unfair if the paladin was a powerful class because of this? Won't that mean all the lawful good mages in the world are getting jipped?
You fail once again.
Nowhere did I say that lawful good grants any kind of bonus or penalty, it's mearly a path your character follows.
Since I apparently have to dumb this down so you won't apply wild accusations to what I post, I shall attempt to.
The Paladin has certain abilities and limitations, one of those limitations is the fact it can only have "one" alignment path.
That means if you roleplay that alignment and class you have a VERY strict path you can follow with very little variation on your actions.
There are certain things you can do, and a vast number of things you cannot since you're "shackled" to the Lawful good alignment.
Nowhere have I stated that an alignment causes a penalty. Being forced into a singular alignment choice "is" the penalty.