Dump size bigger than pgdata size?

["Nicola Mauri" <Nicola.Mauri@xxxxxxx>]

[sorry if this was previously asked:
list searches seem to be down]

I'm using pg_dump to take a full backup
of my database using a compressed format:
     $  pg_dump
 -Fc  my_db > /backup/my_db.dmp

It produces a 6 GB file whereas the
pgdata uses only 5 GB of disk space:
     $ ls -l /backup
     -rw-r--r--  
  6592715242   my_db.dmp
     $ du -b /data
     5372269196  
/data

How could it be?
As far as I know, dumps should be smaller
than filesystem datafile since they do not store indexes, etc. 

Database contains about one-hundred-thousands
binary images, some of which may be already compressed. So i tried the
--compress=0 option but this produces a dump that does not fit on my disk
(more than 11 GB).
I'm using postgres 8.1.2 on RHEL4.

So, what can I do to diagnose the problem?
Thanks in advance,

Nicola