imagine you have a can of soup with the bottom cut off.put your fist in this can( this would be the piston and the can the cylinder) say the can is 10 inches long. you start your fist at the bottom and push up and compress the air in the can(cylinder) to 1 inch from the top of the can. so now you have takin 10 units of air and and compressed it to one unit. this would be a 10:1 compression ratio, do you follow.
welll take the same scenario but make the fist a little taller on top. you will still start from the same postion but the air will be compressed more due to less room between the fist(piston) and top of the can(cylinder head) creating a higher compression ratio. does this make anymore sense??