I did a test restore of JPG files, and they all are about 10% smaller.
I have a large directory of photos (mostly jpg, about 100G), which I had Duplicati back up to an external drive. This completed with no errors or warnings.
I then renamed my original source directory, and restored from the backup to the original directory name.
I then did a diff between the original directory and the restored one, and found that every file on the restored version was about 10% smaller. The file format is fine, and the images appear good to the eye.