[Piwoslaw]

Originally Posted by comptiger5000

Aeromodder - In theory, a high flow can filter just as well. However, it has to be bigger to accomplish this. The more area it has exposed, the better it will filter, and given enough passages for exhaust to pass through (large enough size), it can be effective and flow well.

A catalytic converter doesn't just filter in the normal sense, as a fuel or oil filter does, it uses platinum as a catalyst (hence the name) to convert poisonous exhaust into slightly less poisonous gasses.
But the more area that is exposed, the less that area warms up (spreading the same thermal energy over a larger surface). A cat works only when heated above a certain temperture, that's why cold starts are so bad on emissions. If you are drag racing your car all the time, then the exhaust gasses will be hotter and can warm the larger area, but a hypermiler's exhaust tends to be slightly cooler than normal (since he is using less fuel to do the same job, thus wasting less energy as heat in the tailpipe).
So, the larger the internal area of the new cat, the smaller that chances that it will do its job, especially if you hypermile.