A standard ISO file is a sector-by-sector copy of a physical disc, including the file system and "padding"—empty data (often just zeros) used by developers to fill the disc and improve reading speeds on original hardware. "Highly compressed" versions use various methods to eliminate this excess:
Understanding PS2 Highly Compressed ISO Games Highly compressed PS2 ISOs are optical disc images of PlayStation 2 games that have been reduced in file size to save storage space and reduce download times. While a standard PS2 DVD can hold up to 4.7 GB (or 8.5 GB for dual-layer discs), compression techniques can often shrink these files significantly without losing the original game data. What Are Highly Compressed ISOs? ps2 highly compressed games iso
Recommended for PCSX2 users. You can use tools like 7-Zip to right-click an ISO, select "Add to archive," and choose the GZIP format with "Ultra" compression. A standard ISO file is a sector-by-sector copy
These involve removing "dummy" files or lower-priority data, such as high-bitrate cutscenes or background music, to reduce the footprint further. What Are Highly Compressed ISOs