My unoffical answer is using Mail Flow Central in the unoffical way.
login the MFC mysql
select rcpt_domain,sum(message_size) as total_size from recipients r, messages m
where r.sid = m.sid
and r.mid = m.mid
and timestamp > unix_timestamp(date_sub(now(), interval 1 day))
group by rcpt_domain order by total_size desc limit 0,20
This will give the top 20 total message size processed by receipent domains of last 1 day.
This executes just forever if you want more days or add the transfer_id as a condition..(i left it out delibrately).
There are alternatives,
1.
In my experience, your incoming total size statistics can, alternatively, be generated by your ultimate message store server (sendmail/postfix/exchange).
Your outoging total size is a little difficult if not using the above tweaked method.
2.
scp the mail_logs out regularly, look for "MID XXXXXX ready YYYY bytes for
"
Do some mining scripts.
Chris