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The reason, whether it's right or not, would be that the exhaust gasses get burnt a second time in the cat and therefore should take up less volume. Do a cat back and you'll see some power and mpg increase.
If you do a catback with 2" all the way through, maybe with a straight through muffler of some sort (flowing proper cfm isn't the only thing, a smooth flow is necessary) then you will see your best gains for the money.
Remember, exhaust works with FLUID dynamics. Meaning, think of water flowing through it - is it smooth, or will the water get chopped up and sloshed around?
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