And when you lower the jug or put a taller piston on you are not changing the volume between the top of the ports antop of head. To lessen the volume in this area, you would have to bring the head down, which is very hard to do as the jug and head is all one piece.
Compressing fuel air mixture to just a point before detonation will yield the most power.
Kinda the same as when you have a tired saw with low compression due to worn rings, re ring it and leave everything stock and it runs better again, reason, compression.
I was concerned a while back about some of these high compression ratios on a saw with 93 octane fuel, i think the inefficientcy of a chainsaw muffler design vs a real tuned pipe on lets say a dirt bike or snowmobile is the reason. I have built 800cc snowmobile engines that made 185 hp and over 100 ftlbs of torque. 125 cc dirtbikes were putting out over 30 hp. Chainsaw engines with the muffler can't come close.
Getting back to compression, the 185 hp engine could only have 120lbs of compression, due to when running the pipe design sucked more fuel and air in, we don't have the luxury of tuned pipes on a work saw, so squish the hell out of it!!!