Edit: yes, it’s normal, and it’s true for me too: uploads reported as 1.2G and a backup is 750M. I’ve checked my backups, and they include uploads/default/original but not uploads/default/optimized, which is as expected.
Edit: I see now that “thumbnails” is a misnomer - it’s a collection of pre-processed versions of original images, and can be larger than the originals. See for example