Hi,
When you install netbackup on solaris, it actually gets installed on /opt filesystem.
/usr/openv is just a softlink.
There are many reasons that can account for disk full.
Two big factors in netbackup that consule a lot of space are logs and images.
logs are in /usr/openv/netbackup/logs
and images are in /usr/openv/netbackup/db/images
You can disable the logging as suggested by Taqadus. Unified logging will not be applicable to you since you are in 5.x environment. All the logs of your master server will be located at /usr/openv/netbackup/logs and verbosity will be controlled by "VERBOSE =" entry in the bp.conf
For images, please refer the Admin Guide Vol. 2 which will give you a simple math to forecast the size of your images. It mainly depends on number of clients, frequency of backups and retention level.
Things that you can do:
Either grow the /opt filesystem.
Move the images to a seperate filesystem and create a soft link at original location (not the best idea, but a possible one)
Redirect the images to another location. For this, create the following file : /usr/openv/netbackup/db/images/<client>/altpath
Populate this file with the new location catalog.
hope this helps.