The Openfire Statistic Plugin uses a file called stats.txt for storing all Stats data.
This file tends to grow very fast. The Plugin itself needs data for the last week, so you can do a weekly Log Rotate to control the Size of that file.
ls -al /usr/share/openfire
lrwxrwxrwx 1 openfire openfire 17 Dec 3 2012 logs -> /var/log/openfire
create 640 openfire openfire