RRR,
Post #113, you disappoint me; I thought you were smarter than that. Look at the formula in post #112: Pressure= force divided by area. The force is the force of the expanding gas. The area is the piston area. If the area gets bigger [ a bigger diameter piston ], then the pressure gets less. [ the pressure being pounds per sq inch or whatever medium you are using for measurement.
I will be giving yet another example later using real numbers after I pick up my grandson.