what i can say is..
too small, exhaust gas rushing out,
not enough 'bandwidth' for the exhaust to flow out.
engine is 4 stroke, on the last stroke where the piston push the air out,
it will cost power lost there.
too big,
big diameter, same amount of gas will flow slower.
exhaust will cool down before reach outside,
also cost power lost.
so best is to follow original spec.. or slightly bigger.