Original Wii discs were padded with "garbage data" to fill the physical space of a DVD-R. Modern compression techniques identify this useless data and strip it away. WBFS (Wii Backup File System):
Often used for distribution on sites like Vimm's Lair or the Roms Megathread. Note that you must extract these before your emulator or console can play them. How to Compress Your Own ROMs
If you have a standard .iso file, you can compress it yourself using the Dolphin Emulator: Open . Right-click on the game in your list. Select Convert File . Set the format to RVZ and click Convert .