Indexes growing continuously

Hi,

I'm running PostgreSQL 6.5.3 on Linux-2.2.12 on a 2-way SMP machine.
I noticed that even after VACUUMing there are some indexes that still
remain very big.

For example, here's what I found for pg_attribute:

-rw------- 1 postgres postgres 122880 Apr 4 15:24 pg_attribute
-rw------- 1 postgres postgres 17055744 Apr 4 15:24
pg_attribute_attrelid_index
-rw------- 1 postgres postgres 50176000 Apr 4 15:24
pg_attribute_relid_attnam_index
-rw------- 1 postgres postgres 20758528 Apr 4 15:24
pg_attribute_relid_attnum_index

I know I can fix this by recreating the indexes, but I don't know how to
do it for a system table and if it's safe.

Any hints ?

TIA.

Regards.