Indeed, when big publishers like EA and Ubisoft started leaving Steam they introduced a tiered pricing system which progressively reduces the cut to 25% or 20% after tens of millions of dollars in revenue, to lure those AAA juggernauts back. The price is now indirectly based on how much leverage you have over Valve - Ubisoft can get away with not releasing their games on Steam, so they pay 20%, while small-to-medium studios effectively have no choice, so they pay 30%.

It's especially backwards when you consider that those AAA games put far more strain on Steams infrastructure with their >150GB install sizes.