cylinder volume is a factor in compression ratio...

http://e30m3performance.com/myths/mo...comp_ratio.htm

In that article the writer gives a formula for compression ratio.

He also says "Now if we look carefully at the equation for compression ratio above we note some interesting trends. Even if the volume of the combustion chamber (V2) is kept constant, we can increase the compression ratio simply by increasing the stroke and/or the bore size.

So let's say you rebuild your engine, but all you do is bore it out slightly and install identically shaped, but bigger pistons. You don't shave the head or change the stroke, or decrease the volume of the combustion chamber. You will still increase your compression ratio." Therefore, bigger bore = more compression compared to a smaller bore of the same engine. For example, a 400 at factory bore, 4.125, will have less compression than a 400 with a .030 bore, 4.155. I'm saying this because I just went through it with my own engine.