Is your XI is on a physical server or VM?
This doc is written for XI + VM but applies to physical servers at least as a starting point:
Nagios XI - Running Out Of Disk Space On The Nagios XI VM
https://support.nagios.com/kb/article.php?id=26
If you're running XI in VM refer to our documentation on resizing the VM disk size
http://assets.nagios.com/downloads/nagi ... M_Disk.pdf
What specifically is hogging all the space?
Check whats in /var/log:
ls -l /var/log
You may delete some of the older logs - the ones with a number at the end, i.e. "messages.1", "messages.2", etc. You have to decide you you need to keep, and what you can delete.
Also check the httpd logs:
ls -l /var/log/httpd