db growing out of proportion

Hello everybody,

I'm facing a simple yet gravely problem with postgresql 7.3.2 on x86 Linux.
My db is used to store IP accounting statistics for about 30 C's.  There are
a couple truly trivial tables such as the one below:

CREATE TABLE stats_min
(
    ip    inet        NOT NULL,
    start    timestamp    NOT NULL default CURRENT_TIMESTAMP(0),
    intlen    int4        NOT NULL default 60,
    d_in    int8        NOT NULL,
    d_out    int8        NOT NULL,

    constraint "stats_min_pkey" PRIMARY KEY ("ip", "start")
);
CREATE INDEX stats_min_start ON stats_min (start);

A typical transaction committed on these tables looks like this:

BEGIN WORK
    DELETE ...
    UPDATE/INSERT ...
COMMIT WORK

Trouble is, as the rows in the tables get deleted/inserted/updated
(the frequency being a couple thousand rows per minute), the database
is growing out of proportion in size.  After about a week, I have
to redump the db by hand so as to get query times back to sensible
figures.  A transaction that takes ~50 seconds before the redump will
then complete in under 5 seconds (the corresponding data/base/ dir having
shrunk from ~2 GB to ~0.6GB).

A nightly VACCUM ANALYZE is no use.

A VACUUM FULL is no use.

A VACUUM FULL followed by REINDEX is no use.

It seems that only a full redump involving "pg_dump olddb | \
psql newdb" is capable of restoring the system to its working
glory.

Please accept my apologies if I've overlooked a relevant piece of
information in the docs.  I'm in an urgent need of getting this
problem resolved.

--
Tomas Szepe <szepe@pinerecords.com>