Backup size doubled

[Internet Support]

I am running Amanda 2.4.3 on Freebsd with a single Seagate DAT drive
On a full backup of my /usr directory which df shows the size used
as being 4 gb  I am using nocomp because the client and the server are the
same box.
Amanda for some reason is backing up 8 gb
What's going on? Please help!