OS and NBU version?
From the tuning guide for NBU 6.5
Catalog compression
When theNetBackup image catalog becomes too large for the available disk space, you can manage this situation in either of the following ways:
■ Compress the image catalog
■ Move the image catalog (UNIX systems only).
For details, refer to "Compressing and uncompressing the Image Catalog" in the NetBackup Administrator’s Guide, Volume I.
See here http://seer.entsupport.symantec.com/docs/290282.htm
Control image-catalog compression by setting the Global Attributes property,
Compress Catalog Interval. Use this property to specify how old the backup
information must be before it is compressed. Specify the number of days to
defer compression information, thus users who restore files from recent
backups are unaffected. By default, Compress Catalog Interval is set to 0 and
image compression is not enabled