High Efficiency Video Coding, also known as H.265 . This is the specialized video compression standard that makes such tiny file sizes watchable.
While the engineering feat of putting a 2-hour movie into a 100MB container is impressive, it comes with heavy compromises:
This refers to the target file size of the movie. For context, a standard high-definition (HD) movie rip usually ranges from 1.5 GB to 4 GB. Compressing a full-length film down to 100 megabytes is an extreme reduction in size (often over 95% smaller than a standard file).