wimlib

I created a backup from a Windows 10 volume, that was offline.
Then I restored the image to another, empty volume.
The restored Image on the second volume consumes about 3% more space than the original volume.
To double check, I created another backup of the extracted volume.

Comparing the file lists of both WIM Images showed that they are absolutely identical, as one would expect.

So where does the extra 3% space go?
Both volumes have the same cluster size (4K), so I can rule out that more slack space is wasted.
Could it be that Windows creates hard links upon Installation and that wimlib is unable to recreate these;
therefore making copies of the affected files?

No, it is not hard links, since wimlib preserves hard links, unless it's asked to extract to a filesystem like FAT that doesn't support them.

If you are not using the latest version of wimlib (v1.11.0), the difference could be in sparse files, since older versions of wimlib did not preserved the spareness of sparse files.

Otherwise, there are a number of possible reasons, including the use of "system compression", difference in how space is reserved for files, and other reasons. You may be able to determine the reason if you find and examine some specific examples of files which take up more disk space after than before.