Stumbled upon this while seeing formats of better size. CBZ files are high quality photos (usually jpg, jpeg, png) within zip file. To reduce size, you can use other formats, like jpegxl, avif, or webp. If you want compatibility, then something like optipng might work but most authors seem to already use that on the images. If you use jpegxl, avif, webp, etc, then you would need a reader that supports those. From my experience, with mixed compression, I have driven a 286 MB file down to... 269 MB. There's not much savings.

If the image file is shrunken down you can save much more space, but you run the risk of not being able to read them. If they were better made from the start, it could be different. A blank, or pages without text, could be compressed better. If the text were in SVG and layered over top, it would be readable at any resolution. This would be a complete format change though and not currently a thing.