After installing a service pack you will notice that the used disk space is increased. This because of uninstall files. You can free up disk space by making the service pack permanent, meaning you can’t uninstall it anymore… This also counts for Vista and 7 installations.
Windows Vista SP1: vsp1cln.exe /verbose
Windows Server 2008 SP2: COMPCLN.exe /verbose
Windows Server 2008 R2 SP1: dism /online /cleanup-image /spsuperseded