On the installer image is a directory casper, there is another image filesystem.squashfs, I suspect that it is simply unpacked to the hard disk during installation. System deployment is really fast this way. Also in casper directory are two more interesting files: filesystem.manifest-minimal-remove and filesystem.manifest-remove with a bunch of packages names. And of course you should have a look at the log during the installation process, all the "apt purge" things show up there.