The Internet Archive is a legitimate non-profit library that sometimes hosts older or archived versions of software. Searching for "Subnautica" here can yield results that are safe to download, though compression levels vary.
, there is no official "highly compressed" version provided by the developer, Unknown Worlds subnautica highly compressed download
The ultimate solution to the "highly compressed" problem is to stop storing games locally. The Internet Archive is a legitimate non-profit library