Small file taking up lots of disk space

Hello,

I have a server running RH 7.2 with kernel 2.4.7-10smp.
We are running postgresql 7.2.1 and it has generated a log file.

"ls -l" reports this file to be of 9531465 (9.5MB) bytes in size.
"du" reports this file to be using 17833520 1K blocks (18GB) of disk
space.

I realize there will be some discrepancy between du and ls, but this
is several orders of magnitude.  Just to be sure, I ran "wc -c" on the
file and it reports 9531465 characters in the file.  I then copied the
file to the /tmp directory and then du reports only 9347 1K blocks in
use.  What??!!?

Postgresql is still running while I'm looking at this file, but I
don't think that would cause this.

Anybody have a clue what is going on here?

Thanks,

Jason