Abnormal storage consumption with autovacuum enabled

I'm using 8.1.4 on linux.
Autovacuum is enable with these settings:

stats_start_collector = on
stats_row_level = on
autovacuum = on
autovacuum_naptime = 60
autovacuum_vacuum_threshold = 1000
autovacuum_analyze_threshold = 500

Data stored in the database didn't groove very much in the last months
(pg_dump still generates a 4GB compressed file), but data files now
occupies 20 GB on disk.
I know that a vacuum full or a full dump/restore is needed to reclaim
filesystem space. However, my question is: how this could have been
happened?
I'm quite sure that data in tables never reached that size, during
normal operations.
Could be autovacuum not working properly?
How to diagnose or make sure that autovacuum is doing its job?

Thanks in advance!

Nicola