Step 1. Get 7-Zip here and install it.
Step 2. Right click your
backup folder you copied from your SD -
add to archive.
Step 3. Default settings are fine. Use the
... to specify where the zip will be made.
Step 4. Delete the
backup folder if you want, compressed zip is the same thing just smaller. Depends on what was on your NAND but it saves me 10GB~.