Help understanding VACUUM info on 7.4.5

Hi,

I am running Postgres 7.4.5 and am storing binary objects in the largeobject
table. We want to keep the size of the database and especially the large
object table to a minimum, so we vacuum it (not full) on a regular basis.
However, what we have seen is that even after deleting entries from the
table and vacuuming the free cells don't seem to be being reused. So when we
add a new entry to the table which is smaller in size to a previously
deleted object the DB grows.

Below I have info which shows a DB which has had all objects deleted and
then vacuumed. As can been seen it say it has "9014 dead rows that can't
been removed". What does that mean? If the table is empty why can't they be
removed? Then when I add a new object of ~500k, the DB grows and when I
vacuum again, there are still 9104 dead rows. When will these dead row
become free and available for reuse, so the DB doesn't keep growing?

***** empty pg_largeobject, before vacuum******
bash-2.05b# du -k sql
4472 sql/base/1
4472 sql/base/17141
9060 sql/base/17142
18008 sql/base
136 sql/global
12 sql/pg_clog
32812 sql/pg_xlog
51000 sql
bash-2.05b# !p
psql -d aesop -U xxxxx
Welcome to psql 7.4.5, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

aesop=# \lo_list
Large objects
ID | Description
----+-------------
(0 rows)

aesop=# vacuum verbose pg_largeobject;
INFO: vacuuming "pg_catalog.pg_largeobject"
INFO: index "pg_largeobject_loid_pn_index" now contains 9014 row versions
in 45 pages
DETAIL: 0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.01u sec elapsed 0.01 sec.
INFO: "pg_largeobject": found 0 removable, 9014 nonremovable row versions
in 381 pages
DETAIL: 9014 dead row versions cannot be removed yet.
There were 6230 unused item pointers.
0 pages are entirely empty.
CPU 0.06s/0.04u sec elapsed 0.10 sec.
VACUUM
aesop=# \q

*****store new object in pg_largeobject, before vacuum*****

bash-2.05b# du -k sql
4472 sql/base/1
4472 sql/base/17141
9108 sql/base/17142
18056 sql/base
136 sql/global
12 sql/pg_clog
32812 sql/pg_xlog
51048 sql
bash-2.05b# !p
psql -d aesop -U xxxxx
Welcome to psql 7.4.5, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

aesop=# \lo_list
Large objects
ID | Description
-------+-------------
35509 |
(1 row)

aesop=# vacuum verbose pg_largeobject;
INFO: vacuuming "pg_catalog.pg_largeobject"
INFO: index "pg_largeobject_loid_pn_index" now contains 9148 row versions
in 45 pages
DETAIL: 38 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.03s/0.03u sec elapsed 0.06 sec.
INFO: "pg_largeobject": removed 38 row versions in 36 pages
DETAIL: CPU 0.01s/0.09u sec elapsed 0.10 sec.
INFO: "pg_largeobject": found 38 removable, 9148 nonremovable row versions
in 387 pages
DETAIL: 9014 dead row versions cannot be removed yet.
There were 6079 unused item pointers.
0 pages are entirely empty.
CPU 0.08s/0.16u sec elapsed 0.23 sec.
VACUUM
aesop=# \q

***** after vacuum*****

bash-2.05b# du -k sql
4472 sql/base/1
4472 sql/base/17141
9108 sql/base/17142
18056 sql/base
136 sql/global
12 sql/pg_clog
32812 sql/pg_xlog
51048 sql