It's the same principal as turbo gas motors. I have a turbo 1.8L miata and a 2.5" is better than a 3.0". The reason it's better isn't because it makes more power - it's because you don't get any power gain going to 3.0. So the 2.5 is the same power, plus it's cheaper to make, it's easier to fit and less likely to bang on things.