There is a process called diskmanager running on FMC and FTD that is responsible to cleanup parts of the system so logfiles, databases, etc. only consume a certain amount of disk space. Without any information from the devices themselves it's hard to say if you are running in a monitoring bug or your current version has an issue that causes the disk to get full without diskmanager being able to clean it up.
Can you connect to FTD and execute the following:
> expert
sudo -
df -h
du -s -h /*
du -s -h/Volume/*
