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/