He has said the nozzles are smaller on the 12" so to reduce the pressure and turn it up to the 'sweet spot'. The box says 4000psi and about 31 litres per min so I never thought about nozzle sizes affecting the machine.
I was working on a lower ground patio (Saturday) with access via ladders, so going up and down ladders and over a fence to restart the machine several times was a bit of pain. In the end I had to use the 18"