how to reduce postfix log file size and number of spam bounces..

I've setup a postfix+amavisd-new+spamassassin+cyrus system, for about a
thousand mailboxes and several virtual domains. It is performing
reasonably well, but I notice that postfix generates a large logfile,
with several lines for each message. Is there a config option to reduce
postfix logging to the bare minimum ? Amavisd has one, but postfix..

Also, my mail queue is clogged with thousands of spam bounce messages -
I've got a lot of spam: I believe I've configured correctly my site,
having passed all the open relay tests I could find. I use these rbls:

reject_rbl_client relays.ordb.org,
reject_rbl_client opm.blitzed.org,
reject_rbl_client list.dsbl.org,
reject_rbl_client sbl.spamhaus.org,
reject_rbl_client cbl.abuseat.org,
reject_rhsbl_sender rhsbl.sorbs.net,
reject_rhsbl_client rhsbl.sorbs.net,
permit

I also use the usual body,header,sender,recipient checks to disallow
malformed addresses, 'you are me', and some attachment types.
What can I do to free the queue from bounces to
faked-but-syntactally-regular spammer's addresses to nonexistant
mailboxes on my site ?
I've tried to reduce to one day the queue life and to reduce the bounced
message size, without great results.