It won't compress any better no matter what you do as the data on the CDs doesn't compress well.
It is worth remembering that not all the centos CDs are needed depending on what you select during install time. When asked about what type of system i select Custom and get a bunch of tickboxes to select the packages i want. I tick nothing, i make sure they are all unticked and then use the package manager inside Centos to add any other bits i need. During the actual install it only asks me for CD 1 and CD 2 if that helps any.