It would seem it is common to convert the space to padding used for future tag changes.

http://www.ventismedia.com/mantis/view.php?id=9446

I have seen this problem popping up a few times but couldn't find if a fix exists.
Removing the album art from a .flac file tag does not shrink the file back to its original size. (Don't know for other file types)
The album art tagger script does not seem to do the trick either.
Anyone?