[sdfjkl]

Well, whatever the repackers (e.g. Fitgirl) use seems to work a lot better than what the game publishers use (often no compression at all).

Original Size: 93.1 GB -> Repack Size: from 35.5 GB [Selective Download]

Original Size: 3.1 GB -> Repack Size: 844 MB

And these are lossless, bit for bit reproductions. In many cases assets (audio, video, images/textures) could be using lossy compression for even greater reduction in size with no significant loss in perceived quality (but maybe some loss of advertising/bragging rights).

The downside is, some effort is required packing it: https://fitgirl-repacks.site/new-compression-algorithm/

[SSLy]

Another downside is obscene time it takes to unpack FG releases.

[sdfjkl]

That depends. If you're on a slow connection, the 40 minutes to unpack something are nothing in comparison to the week it took you to download it. If used by the game publisher, the decompression could happen during download (I believe this is how steam does it).

If you're on a metered connection (common in many of the more forgotten parts of the world), the bloated download costs you real money or is simply not possible (or you have to spread it over several periods of data allowance).