Dump size bigger than pgdata size?

[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