XP Volume Migrated to Parallels is bigger than it needs to be

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I migrated an Boot Camp XP volume to Parallels 8. In Boot Camp, my XP OS lived on its own 32GB 32Bit FAT partition even though the OS and all my files only took up 17GB. When I migrated, the XP folder in Parallels is about 30GB or so. How can I make the migrated XP OS folder behave like a Parallels folder in that it only takes up as much room as it contains in files (17GB)?