Hi Picclock
When the steam enters the high pressure cylinder it expands and moves the piston to the end of its stroke , but there is still work left in the steam , so then its moved on to the next cylinder , the steam is now at a lower pressure ,, so this cylinder is LARGER in diameter to get the same amount of work from the lower pressure steam ,,, force = pressure x cross-sectional area .
In the case of a triple expansion engine the low pressure cylinder is working below 1 bar
Rob