Most "highly compressed" files are actually WBFS files. This format removes the "filler" data entirely. You can use the Wii Backup Manager to convert ISOs to this format for use on a soft-modded Wii.
Because compression is computationally heavy, amateur rippers often upload corrupted files. You will get a "CRC failed" error when extracting, wasting your bandwidth. sonic unleashed wii iso highly compressed