Why is my converted ISO file so much larger than the RVZ file?

This is a raw, uncompressed 1:1 copy of an optical disc. Because it contains every sector of the original game disc—including empty data pads—ISO files are universally compatible with almost all emulators and modification tools. However, they take up a massive amount of storage space (usually 1.4 GB for GameCube and 4.7 GB for Wii games).