ORIGINAL: Ian Davidson
A smaller diameter exhaust nozzle would cause the gasses to exit faster – i.e. the only way a fixed volume of gas can exit through a smaller aperture within the same timeframe, is if that gas is moving faster. If the power were to remain constant, then this increase in gas speed must come to the detrimental cost of thrust.
thrust equals mass flow times velocity. At same mass flow by core engine, faster exhaust means always more thrust.
Gaspar