system app - what data is being stored in this index

Hi, Newbie splunk question. I have a Splunk server that is running out of disk space. Looking at all the indexes, I have a "system" app (name = "main") with the homepath of '$SPLUNK_DB/defaultdb/db ' that is occupying most of the space. How can I find out what data is being stored in this index? That will help me decide if I need to reduce the max-size or add more space. Thanks Aspi