Let me just elaborate on that (no offense Manono :)), The black bars exist in every single frame of the movie. To be able to cut the black off, the movie needs to be decompressed (because it doesn't exist as frames in the file, but just compressed data). After decompressing you'll want each frame back into the file which requires compressing again. It's like updating a rar or a zip file, you can't modify a file inside the archive without decompressing->compressing.