Suddenly my OS disk has only 2GB available..

I checked the dir /opt/zimbra and this directory is very large..
The direcotry /opt/zimbra/index is about 1,3 GB
/opt/zimbra/data = 1,5 GB (my messages are stored on other disk, so what is this directory?)
/opt/zimbra/db = 1,8GB

Can someone explain me why index dir is suddenly so big?