how supercompressed iso files are working

how supercompressed iso files are working I am recently seeing "Super Compressed" ISO files which have claimed to compress iso files to sizes < 15 mb, where the original file size is in 4GB+. How is it possible?
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bert [Entry]

"It's possible IFF the data in the ISO is compressible to that size via some arbitrary compression algorithm.

ISOs are just files that contain a filesystem; it can be highly compressible. It depends on

Is the filesystem mostly full or mostly empty?
What type of data?
What type of compression? (lossless? lossy?)
How good is the algorithm?

It is possible to compress repetitive text data very small. 4GB of Apache logfiles probably compresses pretty well. But that same advanced algorithm won't work nearly as well (if at all) on audio/video or photographic data.

It could also be smoke & mirrors. I could make a 4GB ISO file that doesn't actually contain any files, and even though it's 4GB it would compress down to mere kilobytes.

It's like the old saying about lies, damned lies and statistics. Just because someone claims their algorithm can do something doesn't mean you will actually get similar results."