Some uploaders use "dummy" files or "sparse files" to make an archive look legitimate. Once extracted, it might appear to be 60GB, but the files are filled with empty data (zeros). The game will never launch because the actual assets (the world, the characters, the missions) are missing. The Risks of Downloading "Highly Compressed" ISOs Before you click that download button, consider the risks:

7z is a file compression format that allows you to reduce the size of large files, making them easier to store or transfer. In this case, the GTA 5 ISO file has been compressed using 7z, resulting in a 24MB file.