Database size stays constant but disk space keeps shrinking -- postgres 9.1

Dear list,

I have a postgres database, 9.1.3, which shows a fairly constant amount of space used by postgres, but total disk space
keesshrinking. 
If I restart postgres the space on my file system returns.

This cluster is replicated to another; that custer does not show this problem.

There are two main databases on this cluster, both using logging. One is used to accumulate some stats on our systen.

The other is a slimmed-down version of our production database, which gets recreated hourly by a shell script which
pullsdata from remote servers, does a pg_dump of the resulting 3 gig database, and then drops it. 

I posted a message about ths a few days and got no responses. I am hoping for better luck this time as this is a
seriousissue,not relayed t vacuuming or routine admin tasks which may or may not be happening. 

Thanks for any help / advice / things to look for.

Greg

a few details:
db11:5432=# select version();
                                                   version
--------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.1.3 on x86_64-unknown-linux-gnu, compiled by gcc-4.4.real (Ubuntu 4.4.3-4ubuntu5) 4.4.3, 64-bit

Says one of our sysads:
"These are single disk systems. single no
n-raid sata. Using xfs as the filesystem like the rest of the dbs."