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.
But if having cooler, denser gasses in the exhaust is restrictive, wouldn't that restriction prevent overscavenging of the cylinders... wouldn't overscavenging then be more likely with a smaller pipe, if it was indeed less restrictive at low rpms?
