I think this all gets back to eager anticipation of TSM v6 features
being expressed about a year ago, when excitement about a DB2 database
was countered by experienced cautions to "beware what you wish for".
The "classic" ADSM/TSM database was tailored to the product, and
contained efficiencies.  DB2 is a general database, where you can
expect more space utilization in its overall processing.

See 
http://publib.boulder.ibm.com/infocenter/tsminfo/v6/index.jsp?topic=/com.ibm.itsm.srv.doc/c_archive_log_space.html
 for some basic information, and IBM document numbers 7015353 and
7015525 for webcast PDF material on the new database.

   Richard Sims