The short answer is no.

You can do an easy experiment to see this using image files. Grab a random JPG file and open it in a graphics program and save it as a BMP format image.

JPG is already compressed, and BMP is absolutely not compressed. Then try compressing each image. You’ll find that the JPG doesn’t get much smaller, or might even be a bit bigger when compressed. Now do the same with the BMP - that one makes for a smaller RAR!

The main issue here is that compression is about removing empty space in a file (it’s a weak analogy but bear with me). If the file itself already had some kind of compression (basically every AVI or MP4 or MKV you download probably is already compressed), then there’s already a lot less empty space inside the file. RAR doesn’t have much empty space left to work with, so it can’t really reduce the file size any more.

It’s worth doing some testing on a single movie to see how this all works. You’ll probably find that it’s best to just leave the files exactly the way they are. No RAR. No ISO. No tricks. The gains simply aren’t there.

If you’re looking to save on some disk space with your movies, you’d get a lot farther by just deleting one movie you don’t really want that badly. The amount of space you get back from that will exceed your compression gains. It also means you don’t have to go and uncompressed the movies every time you want to watch one.