The primary variable for the piston specification is the compression ratio. Compression ratio, as a measure, is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy.
Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel engine/air temperature and compression ratio all combine to determine compression pressure. Type of fuel used and even altitude also play a part in the decision. Supercharging and turbocharging can drastically alter compression pressures.