Musk explains why SpaceX prefers clusters of small engines
A large portion of the  efficiency of small engines comes from the reduction of &quot;hoop stress&quot;. This is the linear tension in the wall of a pressure vessel(rocket engine) which varies as the square of the diameter. Twice the diameter = 4 times the pressure (hoop stress) the walls must take.
A rocket is a special case of a balloon - with an expansion nozzle attached to couple the impedance of the combustion chamber to the outside environment.
This means that 9 smaller  rocket engines will weigh less than 1 large rocket engine of the same thrust. On top of this is the small engine basic throttling capability with the overlay of cutting out engines to add greater  capability.