huge pg_toast_<OID>_index files?

Hello,

pgsql 7.3.2 on Linux 2.4.18:

I observe huge files (200Mb-1GB) in the database base dir which belong
to pg_toast_<OID>_index relations. The <OID> itself is a pointer to a
user table relation which looks quite fine, not too big, say 2000 rows
with moderate data. These user tables are typically written to very
often.

I tried to vacuum full the corresponding user table, reindex this
table or even drop the indexes to no avail. pg_toast_<OID>_index stays
the same big size. How can I reclaim space on this relation and what
is it used for?

Thanks for help,

Dirk