cooling of exhaust gasses is a big issue, since as a gas cools it does contract, but its density also increases and its this increase in density that restricts flow.
If a larger pipe is used then the ultimate gas temperature will decrease as the gas expands to fill the pipe, not only that but the larger pipe has more external surface area that will radiate heat away and further cool the gasses.
Idealy a pipe size should be selected that will allow the "optimum" gas velocity to occur when the engine hits the RPM at which it makes its max power. If the pipe is too large, at low RPM it will actually over scavenge and may go as far as to pull raw fuel from the combustion chamber which will decrease power and waste fuel.
I believe the SVT stock system from the main cat to the rear split is 2.25" outside diameter, but due to the wall thickness the inside diameter is about 2". Increasing to the next size larger, 2.5", which has an inside diameter of 2.25", which is acutally a big increase over the 2". (remember the equation, pi x radius squared = area of a circle)
cross sectional area of 2" ID pipe is about 3.14 sq. in.
cross sectional area of 2.25" ID pipe is about 4 sq. in.
