twistedenginestarter seems to be missing a key point: The size (thrust rating) of the engine is driven primarily by takeoff conditions - RW length & altitude, ambient temp range, TOGW (ie payload plus fuel)... or perhaps by TOC (top of climb).
After that, the engines are run at whatever setting is needed to maintain altitude and Mach; If it's a short flight, the fuel load is probably light, so thrust required isn't so high. Slowing down the aircraft beyond some optimum point
will not save much fuel, regardless of the engines' rating.
(The neighbor lady always cautioned me to fly low & slow...
)