Niall-
For a given pitch and RPM, more blades or larger prop absorbs more torque hence more HP (or kW) since HP=torque * RPM
For a given pitch and RPM, a larger prop accelerates a greater mass of air while more blades accelerates the same mass of air but to a greater velocity (less 'slip'). Hence, either will give a greater thrust.
Of course, a constant speed prop complicates things since pitch isn't fixed, but the principle holds.