pg_statistic_relid_att_index

Hi all,
    I've a problem with the pg_statistic_relid_att_index system index. The server
version is 7.1.3 (so reindex is not available). This database is heavily
updated,inserted,deleted and thus vacuumed. Since 1,5 year it runned perfectly
but since the beginning of the we've got slow select. In many case, as the
system goes slow, it often a question of table size. So I looked at the oid
sizes in my data directory and the filenode corresponding to the
pg_statistic_relid_att_index get about 87MB of my disk whereas other object
remains quite small. I presume this index is not cleaned while vacuuming
pg_statistic table (with or without analyze).  My problem is a performance
problem. I would avoid to upgrade database to 7.3 (some upgrade would be done
soon and I do not really have time to spend now for such a task) and I would
like to make this index slimmer as possible to get the performance back (for
this database speed is a critical factor).

    Thanks in advance for any help/hints :)

--
Jean-Christophe ARNU