Compression is defined by a taking a fixed amount of air and pushing it into a smaller space. If you vary the size of that space, you get more or less pressure, given the same amount of air. Most gasoline engines are in the 9-13:1 range, diesels are 16-20:1 or more. They are taking 9-20 units of air, and compressing it into the space of 1 unit.

This is why installing thinner head gaskets or shaving down a cylinder head is sometimes done for added performance, as it increases the compression ratio. Shrink the combustion chamber, get a bigger bang.

If you wanted to get a ballpark guess of the what a cylinder should be making, you can multiply the compression ratio by 14.7 (atmospheric pressure at sea level). My 8:1 example above would be close to 120 psi in perfect scenario, unlikely to hit that exact number in the real world, but it should be close. Drop it to 6:1 by use of a long adapter, and you're looking at 90 psi, even though the cylinder in question is likely capable of more.

As Pappy pointed out, it's a bit of a moot point since it's only half of the equation in a two stroke, but worth noting IMO.