The Wii’s SD card slot is limited to 32 GB (SDHC). A full 4.37 GB ISO eats up 15% of that. By keeping a highly compressed archive on your PC, you can swap games in and out of your SD/USB without keeping massive ISOs lying around.
If you manage to obtain a legitimate backup, it is worth noting why PES 2013 on Wii is unique compared to the PlayStation 3 or Xbox 360 versions: pes 2013 wii iso highly compressed