Kok Wai,
70GB is indeed unusually large. Our detailed Delivery Logs are @ 60-90MB for @ 60,000 emails/day (and about 3-4MB compressed). Unless your SmarterMail is delivering @ 46 million emails/day then there is something definitely amiss with that file size.
I believe that version 11 of SmarterMail allows you to set SETTINGS > LOG SETTINGS > COMPRESS LOG FILES AFTER XX DAYS. (Can't remember exactly which version this feature was added.) We routinely set this to 3 days, resulting in even our detailed IMAP Logs which exceed 2GB in size to be compressed to a much more reasonable 60-70MB.