Big difference in databasesize compared with disksize

Hi all,

Problem:
If I use
SELECT pg_size_pretty(pg_database_size('dbname')) As fulldbsize;

  fulldbsize
------------
  6455 MB
(1 row)

If I compare this output with
/var/lib/postgresql/9.0/main/base/[DBFOLDER]
I have a difference of 3 GB, which I think is not normal.

To make a little bit clearer what i mean:

/var/lib/postgresql/9.0/main/base/43169# du --apparent-size -h
6.4G    .

/var/lib/postgresql/9.0/main/base/43169# du -sh
9.4G    .

Is such a big fragmentation a normal behavior? It's splitted to several
Filenodes.

I also made a "xfs_fsr" to defrag all of this, i gathered around 1 GB
back ( would be more if the tables wouldn't be written in this time )
But within some hours the fragmentation was back..

There is also no big dead tuple count in this database, it's getting a
vacuum analyze every night.

Right now i am out of ideas... What do you think i could do?
If you need more information i can provide.

Environment:
- OS: Debian Lenny
- Disksize: 15GB
- Filesystem: XFS
- Psql: 9.0.3

thanks in advance.

regards
Bernhard