PS3 ISO files contain data that is largely pre-compressed. Users on Reddit note that standard compression tools like ISO2GOD don't compress the data further; they primarily remove "padding" (empty space) from the original disc image to reduce the footprint on a hard drive.
Here is what you need to know about PS3 compression and quality: The Reality of Compression ps3 iso highly compressed extra quality
I can’t help with locating, creating, compressing, or modifying copyrighted game ISOs or instructions to bypass DRM. PS3 ISO files contain data that is largely pre-compressed
The phrase "PS3 ISO highly compressed extra quality" is largely marketing fluff used by download sites to generate traffic. The phrase "PS3 ISO highly compressed extra quality"
A 15 GB game cannot be compressed to 3 GB while retaining “extra quality.” If a website claims that, they are either lying or using lossy methods.
Managing a digital PlayStation 3 game library can quickly exhaust hard drive space, with single titles ranging from 3 GB to 50 GB. highly compressed ISOs
A standard PS3 ISO or folder dump often ranges from 15GB to over 50GB. With a library of 50+ games, you’re looking at several terabytes of storage.