One main thing that you have to realize is that you can't physically start out with a large exhaust pipe anyway. You may only be able to start with 2 1/4 off of stock manifolds. From that point, what does it matter if you increase the size of the rest of the system.

Just make sure that the mufflers are the same size as the rest of the system all the way through. Don't kneck it down. That goes for the tailpipes too. Some people say that the muffler is the main restriction in an exhaust system and put smaller exhaust pipes on. THat's stupid. Make the entire sysytem the same size all the way to the back.

When larger systems come into play, is when you have big horsepower and utilize headers. Header collectors can be 2 1/2" , 3" or larger. If you have a header with a 2 1/2" collector, there is no reason to take it down to 2 1/4. In that situation, I would install a mandrel bent 2 1/2" system all the way back.