If your information store displays unexplained rapid growth, it's probably the result of a rule that's creating a loop. To find the problem mailbox, follow these steps:
Once you've located the culprit, log on to the mailbox, and delete the faulty rule. You'll probably want to clean the mailbox of the excess messages as well.
If this loop has gone on for a while, your information store may have grown very large. It's not unheard of for the information store to increase by several gigabytes over a period of a few hours, depending on the nature of the looped message.
If the growth is enough to cause disk space problems, you may need to perform an offline defrag to regain the space. Setting storage limits on your mailboxes is one way to keep a similar problem from getting out of control.