Your compression level might be too aggressive, or the file is corrupted. Try changing your emulator's backend from Vulkan to OpenGL, or redownload the file.
Here is where the "highly compressed" scene comes in. Gamers use specific tools to rip out unnecessary data (like firmware update files or unused language packs) and then squeeze the remaining data into much smaller containers. A raw ISO might be around (over a Gigabyte), but a "High Compression" file shrinks it significantly.