First, I'd like to say thanks for the input.

And this is a brief extract from what I understand now:
a)  ADSM does not track the usage of DB backup tapes and
    EXPORT tapes in the DB - this is just part of the
    VOLHIST. ALL other tape usage is also tracked in the
    ADSM DB.
b)  The VOLHIST records can - and should - regularly be
    deleted. One should keep in mind how long the DB
    backups and the EXPORT tape info's are needed and
    delete the rest up to let's say 3-5 days ago.

The result is:
a)  Still usable VOLHIST for Disaster Recovery.
b)  Smaller VOLHIST file which also increases the
    performance of deleting a tape volume (cause unknown
    but I suspect that this VOLHIST is read and sorted in
    memory when a volume is deleted).
    Side effect: ADSM complains now that the volume can't
    be found in the VOLHIST with "ANR9999D ASVOL(nnnn):
    Unable to delete scratch volume vvvvvv.".

Best regards

Andreas Peikert, Systems Programmer
   aMaDEUS Data Processing GmbH
   P.O. Box,   D-85424 Erding,   Germany
   Fon: +49 (0)8122 43-5805
   Fax: +49 (0)8122 43-3260
   email: apeikert AT amadeus DOT net

P.S. Kind greetings from Bavaria to Austria, too!