F3G
The 800 is more like a 757-100 than a 737-300. If you see the 800 parked next to one of the classic 737s you realise just how big it is.
FR have 26K CFMs which can be derated via the FMC to 24K and 22K: essentially it is like unbolting the 26K engine and fitting a less powerful one, as the selection lasts for the whole flight. The engine can then be operated at reduced N1 by using an assumed temperature - fooling the engine management computer by telling it the air less dense.
At very light weights, less than 55 tonnes it is very quick, even with a derate and assumed temp: if the runway is short, a la Belfast City, then 24K will probably be used and it goes off like a rocket.