Is autovaccum lazy?

Hello.
I just ran these commands

select pg_database_size('cdb');
vacuum full;
select pg_database_size('cdb');

and got

315250448
----
90669060

So I guess that the vaccuum full command released space that was not
used any more. A week ago I guess the same commands would have
resulted in quite similar numbers. Is there some way of finding out
what the autovaccum have done? I'm having a hard time beliving that
the autovaccuum has done it's job properly since there were so much
"wasted" space, or did it just not find it worth the effort to release
the unused space?
Any thoughts about this?

I might have a similar system I could run these commands on. Any
advice on other commands to run then to find out more?