Data Partition is Growing

I'm an application developer trying to understand a pg data partition
issue. We are storing large objects (gifs) at a rate of about 1GIG /day.
After about a week, most of the objects are recycled -- that is, an
insert triggers a delete of a similar object. The postgres data
partition, though, continues to grow, even though the objects are
successfully deleted from the application *and* largeobject tables. I
understand that removing an object from the database does not
automatically free up disk space, but I expected to see the disk space
"level out" at some point. The data partition is currently at 40Gig and
growing.

We are running Postgres 7.4 with the incremental vacuum process active.

Any thoughts?

Thanks,
Mike