> I am wondering how to compress .cbr and .cbz files
`cbr` and `cbz` stand for `Comic Book Rar` and `Comic Book Zip`. Those files are basically JPEGs stored in a archive with a custom extension. Since they are already compressed in a archive you cannot reduce their sizes by archive compression.
What you do can instead is to extract the images from the archive with `7z e file.cbz` for example and process the JPEGs to reduce the image quality (and size) with the `convert` command from ImageMagick. Finally, you can rebuild your archive with the `7z` utility or the `zip` command (or alternatively the `rar` command if you want `.cbr` files). You just need to rename your `.zip` files to `.cbz` (or your `.rar` files to `.cbr`).