Bottom line, winrar isn’t the tool to compress video files. In short it’s more complex, but zipping, raring etc… those methods are all the ideal way to compress executables, word documents etc… In short, most likely your video files are already compressed as much as they can be without loss of quality. However if you were to attempt to make them smaller, most likely you’d use something like handbrake or some other video codec converter to actually try to shrink them.