vacuum process size

Just for a testing I made a huge table (>2GB and it has 10000000
tuples).  copy 10000000 tuples took 23 minutes. This is not so
bad. Vacuum analyze took 11 minutes, not too bad. After this I created
an index on int4 column. It took 9 minutes. Next I deleted 5000000
tuples to see how long delete took. I found it was 6
minutes. Good. Then I ran into a problem. After that I did vacuum
analyze, and seemed it took forever!  (actually took 47 minutes). The
biggest problem was postgres's process size. It was 478MB! This is not
acceptable for me.  Any idea?

This is PostgreSQL 6.5.1 running on RH 6.0.
--
Tatsuo Ishii