If you use TSM on AIX, you might look at www.servergraph.com. It has a piece
that tracks the activity log's "client X backed up 4.56 MB" messages and
writes a daily total. Costs money, but there's a free trial.

or you could go through the activity log manually and parse out these
messages...it's ANE4964 (or ANE4961?)

or you could go through the dsmaccnt log and dig it out that way.  The admin
guide has the dsmaccnt file layout - look up "dsmaccnt" in the index.