science over load !
original question, compression ratio is size of cylinder/gasket space/piston dish/combustion chamber in head...when the piston is at the bottom of the cylinder. divided by the size of those areas at top dead centre.
so in a 9:1 engine, the space is 9 times bigger at bottom of the stroke, to the top of the stroke.
what makes it higher or lower compression, you reduce the size of the combustion chamber in the head (or not have one at all like some diesels) or make the piston bigger with the chamber size unchanged.
to some degree its by the by as the cylinders only every fill to about 70-80% capacity (normally aspirated engines) and at a narrow point in the rev range, so the actual compression ratio (called dynamic compression) is allot less