We experienced the same issue after upgrading from 7.1 to 7.5.0.4 (We're running AIX Master and media servers), and leverage BMR.
What Symantec found out was TIR information was not being pruned during the image cleanup processes. (Our catalog grew from approximately 750 Gig to now just over 1.2 TB). We're testing an EEB which does appear to have resolved the issue. (EEB 3108642 which replaces BPDBM).
What we found in our bpdbm logs which pointed to the TIR pruning issue was (grepping for compress in the bpdbm logs):
01:18:05.061 [13435048] <4> ImageDelete::~ImageDelete: deleted 4404 expired records, compressed 2198, tir removed 6, deleted 5922 expired copies01:18:05.061 [13435048] <4> ImageDelete::~ImageDelete: deleted 4404 expired records, compressed 2198, tir removed 6, deleted 5922 expired copies
Since we backup, using BMR, approximately 1700 clients per night, that led Symantec down the road of looking at the TIR info.
Not sure if this relates to you, but thought I'd pass it along, with relevant EEB info so you can mention to support. :-)