about the performance of autovacuum and vacuumdb?

i try to compare two methods of garbage-collect.(Postgresql Ver:8.1.3)

1. enable autovacuum without using vacuumdb -f mydb in crontab.
2. using crontab to vacuumdb in every 10 min (autovacuum disable)

the method 2 got more free space than method 1.

the disk size of database still  increased in method 1.
so, what's the advantage of autovacuum?