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