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).
