Duplicity fails on 3GB /tmp

Backup application Duplicity has just filled up my /tmp, located on / partition with 3GB free space, during verification of a full backup and, finally, reported 'failure'!
After quitting Duplicity, I have 1GB less free space on my / partition.

I wonder how much bigger the /tmp space need to be...


Update [2013/11/19]:

I've found the 'lost' GB into ~/.cache/deja-dup directory [naturally] (or /root/.cache/deja-dup if it runs under gksudo), but I still don't know why 3 GB of temporary space are not enough.