Have you tried this before suggesting it? I was able to compress the linuxmint-14.1-cinnamon-dvd-64bit.iso file by 0,68% with 7z (reduced from 881 MiB to 875 MiB, or a 6 MiB reduction). That doesn't seem very significant.
Also, it would mean that anybody downloading the ISO would need to be able to extract .7z files. I'm not sure that Windows is able to extract such files, or OS X, or for that matter Linux distros that don't have p7zip installed. Extract such a huge file is slow, and would mean that instead of 881 MiB disk space for the ISO file, you would need almost double that because you can't delete the .7z file until you have extract the archived ISO file from it...
I'd recommend instead you use either OSDisc.com
to get Linux Mint on DVD or USB stick. Both are Linux Mint partners, and contribute a portion of their sales back to the Linux Mint project. So you'd be supporting the project as well! And you can share the DVD or USB stick with others in your area that want Linux Mint but don't have a broadband or unlimited data subscription.