This perceived increase is also offset the "standard" way, which is by reduced damage output and hitpoints gained.
Cleric: Damage d6, base HP 6
Wizard: Damage d4, base HP 4
Fighter: Damage d10, base HP 7
Thief: Damage d8, base HP 5
It all seems to work nicely with the "old school" feel of Wizards, and to some degree Clerics, being relatively weaker at level 1, but a level 10 Wizard or Cleric is very powerful by comparison to non-casting classes. Contrast with DnD 4E where the power progression between casting and fighting classes is more stable.
However, I really like the notion Sage presents where non-casting classes gain access to more special options, while casters basically gain more options with regards to their casting. Really, what it's about is not becoming more powerful as you level up, but gaining more options - this makes you more adaptable, and leads to continuously fresh gameplay.