you're getting confused between capacity(swept volume + CC volume + HG space) and the compression ratio. The volumes are the same on both cars, but the compression i.e. the volume available at TDC is less due to the dish in the 3 door piston, thus is doesnt squash the mixture as tight into the same space. The volume of the cylinders is usually only refered to as the swept volume. The cars do have different comp ratios but be sure that the dish is very small in real terms. Without really taking good measures you might find the dished piston actually has a thicker crown hieght around the squish band which could equaly out the comp, but I pretty sure the series 2 actually had lower comp. I used 5 door pistons on my first engine for that reason.
