Thermo-dynamics says other wise:
3" dia, hot gas, PV = nRT, as T drops the other side of the equation needs balance, easier to reduce volume by dropping pipe diameter, keeps pressure and gas velocity constant, helps reduce bacpressure, helps get exhaust away.
Alot of people arque both sides, at one point we made a MatLab flow model that shows the 3" -> 2.25" step to work better, I gotta find that.