The Faststone Photo Resizer will let you shrink the size of the jpegs in any .cbz file and there are converters available to convert a .cbz file to EPUB, for instance - some you can do online and others can be downloaded. However, I think the output EPUB file will be a set format and might be difficult to read on a basic e reader - Nook, for instance. But I have to ask, why are you concerned? Are you reading on a tablet, pc, laptop? If so and you are concerned about storage, delete it after you have read it. If you want to keep the file, use cloud storage if on an Apple device, or on a micro SD card if on an android device, and there is the option of investing in an external hard drive. If it's of any interest, I have a couple of micro SD cards for my Samsung tablet and I switch files from an external hard drive on and off the cards depending what I want to read.
Curious to know more about your problem.