Original Wii discs contain "padding" or dummy files meant to fill the 4.7 GB capacity to improve read speeds on physical hardware. Compression techniques target this unused space:
WBFS was designed specifically for Wii game management. Unlike an ISO, which is a sector-by-sector copy of the disc (including empty padding data), WBFS is a "scrubbed" file system. It extracts only the actual game files, stripping out the dummy data and region-specific padding. wii highly compressed games