Well it really isn't so much to do with displacement as it is the port timing of the motor.
The timing of a .21 vs. that of a .28 are very similar, and the design of the pipes are "tuned" to the exhaust pulses of the engine. The timing of the pulses are more tuned to the exhaust.
Most of the exhaust is sent back into the engine, that it how a expansion chamber works.
Beleive it or not, that small hole in the end of the stinger is big enough to flow a .28 engine