The latest version of Mp3tag (do not know how it was in earlier) after removing the cover from the FLAC file's size is not reduced, and it should. This problem does not occur in the case of MP3 files. Regards
There is a German thread from 2012
Größe von FLAC-dateien wird durch entfernen von Album Covern nicht reduziert
which points back to an English thread in 2011.
So apparantly, this is no new behaviour.
Thank you very much for your response. It is interesting that when I remove the cover using the AIMP 3, the file size is reduced ...
The referenced thread mentions the program Metaflack to reduce the padding. Perhaps that is even better for your purpose.