Big index sizes

We have serveral table where the index size is much bigger than the
table size.

Example:

select count(*) from product_price -- 2234244

Table size: 400 MB
Index size: 600 MB

After executing "reindex table product_price", index size reduced to 269MB.

I believe this affects performance.

Vacuuming a table does not rebuild the indexes, am I right? I'm not sure
if I need to do this manually, or is this the result of another problem?
(For example, too many open transactions, frequent updates?)


Thanks