Originally Posted by Nixlimited

I don't know if it's necessarily an issue of flowing "too much." Turbos run off air flowing through them and the more air, the more force on the turbine blades, which then turn the compressor wheel. I think the reason you may see such small exhaust piping for the rear mount is to maintain the speed of the exhaust so that it imparts more energy on the turbine blades. As the exhaust diameter gets larger, there will be less back pressure, and the exhaust will slow down and spread out in the additional space. With a huge exhaust going to a rear mount turbo, I am guessing you would have a tough time spooling the turbo.