Compression uses algorithms (like ZIP, RAR, or specialized tools like FreeArc) to reduce file size. A "highly compressed" PC game might compress 50GB down to 15GB by repacking audio (downsampling) or video (re-encoding). However, of random data (like encrypted game code) yields very little size reduction.
If your PS4 is not on firmware 9.00 or lower, you cannot play any downloaded games, compressed or not. For 99% of modern PS4 owners, highly compressed ISOs are completely useless. ps4 iso games highly compressed