Engine needs backpressure to work?

Now a day is common to see motorsport lover modify with bigger exhaust muffler on their vehicle, some people confused with louder and higher flow exhaust system could make the car more power or faster, in fact a larger size exhaust pipe and muffler might not suitable for all type of vehicle specially on new model car with ECU, data from Air flow and MAP sensor is directly affect the Air fuel combustion in the cylinders, generally internal combustion is the major process that generate power/torque for vehicle motivation.

Theoretically an oversize exhaust system caused leak of backpressure,affected Imperfectly combustion due air fuel mixture over flow before completely combustion in the cylinder block caused lose torque on low/mid rpm range, however does not
automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture.

Overview:
Increase of exhaust piping size only when internal combustion exhausted additional waste gases needed for highest velocity because of engine modfication such as:
-Stroke Kit modification
-Bolt on Forced induction
-Changing to performance camshaft
-Any other modification that effected exhaust valve timing or air fuel mixture capacity volume
Idea size to be increase: