QuickTime 3 Reference
Your software can read movie files that contain compressed movie resources. Compressing a movie resource can reduce the size of the resource by as much as 50%.
Applications or other software do not have to be changed to use compressed movie resources. They must, however, use QuickTime 3 or later; earlier versions return an error when attempting to open movie files that contain compressed movie resources.
An application can create a compressed movie resource when flattening a movie by calling the FlattenMovieData or FlattenMovie function with the flattenCompressMovieResource flag set. This compresses the movie resource stored in the file's data fork, but not the movie resource stored in the resource fork. Compressing just the data fork is sufficient for most purposes, including transmission of movies over the Internet, where only the data fork is used.