WBFS (Wii Backup File System) is a file format used to store Wii and GameCube games on a USB drive or SD card. While Sunshine was a GameCube title, it runs perfectly on a modded Wii that supports or USB Loader GX .
A standard Wii or GameCube ISO is a 1:1 copy of the disc, often 4.3GB or more, regardless of how much actual game data is inside. The WBFS Solution: Super Mario Sunshine Wbfs