Never experimented with overly large pipe but I know in HVAC, too large of duct work creates tumbling of the air flow which can cause more noise and slow down the flow (back pressure?). You wouldn't want to install 3" pipe on an engine that can't use pipe that big and you for sure wouldn't use a pipe that's too small. IMO, a smaller but correct diameter pipe would create a better quality flow through it but getting too small will create back pressure and that's not what you want to do. I think this is what many think is going on when they go with too large of pipe and call it back pressure.....