WHOA! science content is definitely in question throughout this thread. I could go on to explain, but it seems like this would give you enough of the information that you need.
Exhaust System Theory 101
If you read through it, you will see that back pressure, which will occur in low diameter tubes at higher rpm, is not what increases the torque, but the exhaust scavenging that happens due to the fact that your combusted air/fuel , in fact, has enough mass that at the velocity (e=m*v*v) it has significant momentum. This, coupled with overlap of the intake and exhaust lobes with each other, and the trail of that lift into the surrounding strokes, increases volumetric efficiency, which increases torque at that rpm. This is not the same as welding your 5" muffler shut to a 1", which would increase the back pressure, but not the velocities that the exhaust would be seeing in the more critical sections that effect this scavenging effect. You also should look up what an x pipe does (hint, increase cross scavenging between the two banks of cylinders) along with tri-y headers. A good rule would be that as temperature drops and density increases, diameter should as well. An exhaust system that has such a taper would be expensive to build, hence why we don't have them. This only begins to tell the story. I hope this helps.