Usually Qmail queue is growing due to too many messages in remote queue, because sending mail to remote servers takes much more time than delivering the message to the local mailbox.
By default Qmail uses up to 20 'qmail-remote' processes to deliver mail to remote servers. This value can be not enough if the server has heavy mail traffic. In this case you can increase number of simultaneously running 'qmail-remote' processes by creating /var/qmail/control/concurrencyremote file with needed value, for example:
# echo 50 > /var/qmail/control/concurrencyremote
Then restart Qmail. See "man qmail-control" for more information. Also, see the articles about SPAM issue 766.