Confused about database overhead

I recently switched to 3.5.7. I created a new database and then uploaded a CSV file with the email addresses. Thanks to some help from Duncan Cameron, the mailing system is working brilliantly. Here is my confusion. The new database is tiny - only a little over 11MB. The old database from which I took the emails was 640MB. I can’t help but think it was hacked and used by someone else although the only emails that downloaded from it were those of my customers. Any ideas about how the database grew so large and how I prevent the new one from taking on all that bloat?

@fmw Commonly a large database is caused by the bounce table accumulating records and those never being deleted. Depending on how you deal with bounces, they can deleted on the View Bounces page.

Also, depending on the number of subscribers and frequency of sending campaigns the usermessage and user_message_view tables can also get quite big.