For users playing via USB, files must often be processed through tools like

PS2 games, stored on DVDs, have large file sizes, often up to several gigabytes. A "highly compressed" ISO is simply a game file that has been compressed to take up much less storage space on your computer or device. You will often see these files with the or .zip extension. These files work much like a suitcase: you need to "unpack" them before you can use the game. For emulation, the widely recommended format is often CHD , which uses a mix of LZMA (for game data) and FLAC (for audio) to achieve the best compression rates without losing any quality.

When you search for "Highly Compressed," you are looking for a version of the game that has had unnecessary data removed or been compacted to save file size.