Standard GTA SA saves do not work on highly compressed versions because the memory addresses (the "map" of where files go) change after stripping content. You might get 50% through the game only to find you can't start the "Heist" mission.
The original GTA San Andreas installation files are roughly 4.7 GB. A "highly compressed" version reduces this size to approximately 400MB to 600MB. This is achieved by:
Third-party executables included in these packs often lack valid digital signatures and may contain malicious scripts.
No. Do not download the random 400MB version from a shady website.
The 400MB version is an unofficial, modified release where the original game files are heavily compressed. To achieve this small size, creators often: