XFS – Disk Usage is 15-30% Bigger Than the Files

xfs seems to use 15-30% more disk space than the files require:

# du
0       .
# dd if=/dev/zero of=test bs=1M count=5k
5120+0 records in
5120+0 records out
5368709120 bytes (5,4 GB) copied, 10,527 s, 510 MB/s
# ls -l
total 8388608
-rw-r--r-- 1 root root 5368709120 Oct 19 16:04 test
# du
8388608 .

It seems to do that to a certain degree for the whole file system. This filesystem reports disk full when adding files less than 1% of the file system's size:

# df
/dev/sdb6  40957913088 35624042728 5333870360  87% /xfs-export

Is it caused by some fancy option when doing mkxfs (similar to the reserved 5% in ext2)?

# uname -a
Linux server 2.6.39-bpo.2-amd64 #1 SMP Tue Jul 26 10:35:23 UTC 2011 x86_64 GNU/Linux