If you do the math, The volume of the compressed area increases a little, the volume of the cylinder when the piston is at BDC increases a lot (relatively). So the ratio of compressed volume to the uncompressed volume gets a little higher. It isn't a whole lot, but it is increased.
V=Pir^2xH
Assume a 0.125 height at TDC on a 3 inch bore and a 2.875" stroke so height is 3" on BDC
r=1.5
V1 = 0.8835 CI TDC
V2 = 21.205 CI BDC
Compression Ratio = 21.205/0.8835= 24.001
Bore out 0.2 inch 3.2, r=1.6
V1 = 1.005 CI TDC
V2 = 24.126 CI BDC
Compression Ratio = 24.126/1.005 = 24.005
The difference is small, like I said, but it does increase. As the height for v1 gets smaller, the ratio gets bigger (think thinner head gaskets).
As the bore gets larger, the ratio gets larger
as the stroke increases, the ration gets larger.
Cylinder head area affects this too, but assuming a fixed cylinder head area, the above is true.
