If evasion is actually simply your chance of avoiding any given attack, and defence is a fraction the damage is reduced by, both seem virtually worthless in the early game.
I suspect that maybe you realized they become extremely powerful at high values, so you wanted them to grow slowly. But now their usefulness curve makes them practically irrelevant early and then late game (if not limited) they suddenly shoot up to damage immunity.
There are functions out there that exist to help make their usefulness more linear with their numerical value. Basically what you care about is their impact on effective hitpoints (EHP).