People dont relize that larger exhaust pipe diameter can lead to more restriction, or backpressure. Bigger isn't better. If the exhaust is too big, the gasses will lose velosity, and will then lose the energy to push the rest of the gasses out, and then lose the pulse that actually pulls exhaust out of the combustion chamber. It's like trying to push into a lake vs. pushing downstream.
Think about it, when the exhaust valve opens the gasses come rushing out at a high rate of speed, then the valve closes and the pulse of gasses (remember that even gasses have mass) keep moving and effectivly cause a negative pressure in the runner and depending on runner length, that pulse will pull gasses out of the next runner at the collector. If the runners or system is too big, it will lose this pulse. Same goes with intake.
Think about it, when the exhaust valve opens the gasses come rushing out at a high rate of speed, then the valve closes and the pulse of gasses (remember that even gasses have mass) keep moving and effectivly cause a negative pressure in the runner and depending on runner length, that pulse will pull gasses out of the next runner at the collector. If the runners or system is too big, it will lose this pulse. Same goes with intake.