A "highly compressed" PS3 ISO is a game image file that has been processed through data compression software (like RAR, 7Z, or specialized ISO tools) to reduce its archive size for sharing and storage.
To play these games, you must successfully extract them back to their functional state. Required Tools Ps3 Iso Games Highly Compressed
If you are using RPCS3, always update to the newest build. Emulators continuously update their compatibility layers to handle compressed textures and custom ISO structures much more efficiently. Conclusion A "highly compressed" PS3 ISO is a game