There are legitimate methods of compression, such as the "repacks" released by trusted groups, which can shrink the download size significantly by compressing file assets. However, even these high-efficiency repacks still result in a download of approximately 40 to 60 gigabytes. The difference between 100 GB and 50 GB is a feat of engineering; the difference between 100 GB and 100 MB is a lie.
Most of these videos are fake. The creator records themselves downloading a 100MB file, then cuts to a pre-recorded clip of GTA V gameplay. They use clickbait titles and thumbnails to get millions of views. Every view earns them ad revenue. They don’t care if you get a virus.