It sounds like you have already succesfully backed up Windows to the pen drive. It may have need 16Gb for temporary files while it was making the final image, which will be smaller.
But you cant "open" the file on the pen drive as it is probably in some sort of packed format.
If you wanted to use this file on the pen drive you would need to restore from it (see the Windows help for backup and restore).
DONT restore from it unles you need to as it will overwrite all the files on your computer.