delericho
Legend
That's a bit naive though. They do gain more spells, but their existing spells don't increase in DC and become less useful over time.
The thing is, as they go up in level the Wizard will typically change the use of those low-level slots. They drop the classic offensive spells (which, as you rightly note, become less useful), and replace them with either save-free debuffs against the enemy or, more likely, with buffs for themselves and/or allies. Sleep disappears, but it is replaced with nerveskitter, which never loses its benefit.
Of course, that also neglects the role of magic items - through the use of wands and cheap scrolls, a Wizard who is so inclined can effectively bypass the Vancian casting that is the primary balance for his class.
But I'll agree that my analysis of the almost-linear Fighter vs the at-least-cubic Wizard is certainly not exactly rigorous. For all that, though, I am reasonably convinced that the conclusion is sound - Wizards advance at a much steeper rate than the Fighter.
YMMV, of course.
(I should also probably note that when dealt with at the table, I've not seen huge problems. My most recent campaign ran from 1st to 15th level, with a party consisting of a Ranger, a Rogue, a Cleric/Fighter, an Artificer, and a Wizard (Diviner). For a while, we also had a Druid and a Psion. In that campaign, all the characters were able to contribute meaningfully, and right throughout the campaign. It was obvious that the casters were both more powerful and more flexible... but it was never game-breaking.)