index keeps on growing

Hello all,
    I've a hard working pgsql db running on 7/7x365 server. Each night (when
activity is less intensive) I run vacuum and vacuum analyze on tables where
principal activity is performed.
    Tables are cleaned (files sizes are decreasing), but indices files keeps on
growing in size....

    I run PgSQL 7.1.3, I cannot upgrade (immediately) to 7.2.x... So what can I
do to keep these indices' size at a constant and reasonnable size?

    Thanks a lot

--
Jean-Christophe ARNU
s/w developer
Paratronic France