Thanks, yes, I just mailed the bug report as requested. Command line (Terminal) sees 40G (not a very precise number I must say) as opposed to the 43.342.135.926 bytes Finder sees and still sees after restarting it. The difference after this Finder restart is that now it displays two Backup folders of equal size but different name.
In case it matters: I previously attempted synchronizing the database in question via bonjour with another mac, which failed apparently because a (temporary?) file in Application Support (in Library) outgrew the available hard drive space, possibly due to repeated connection failures (despite—as far as I can tell—uninterrupted wlan). Synchronization this way never succeeded, the database kept being categorized as not yet synchronized. It is only after noting the enormous Application Support file that I aimed at cutting down the size of that particular database by taking out a large number of large files from it, only to see that this lead to no shinkage in size at all.