When you see it generally refers to a repack that, once installed, will expand to roughly 2.5 GB to 4 GB . This is a 90% reduction in download size.

Some repacks come pre-integrated with "must-have" performance mods like Loading Screen Mod to help manage RAM usage. How to Install Safely

: Often, "highly compressed" versions under 1GB achieve their size by removing "non-essential" files: