There are no logs in the database. The database contains all the workflow definition and starting rules as well as the state and history of every running instance and any recently completed instances. This is done so the user can search for running instances and see what step they're in, how the conditions evaluations went so far and so on. Completed instances are removed from the database on a schedule. By default completed instances are removed 30 days after completion, but the value is customizable through the Workflow Admin Console.

You can run a statistics report from the Designer or the Admin Console to get instance counts and their statuses in order to get an idea if the size is realistic or if, like Chris suggested, you may have a lot of terminated workflows or an infinite loop. There is no formula for calculating the size because it varies with the size of the workflows and their expected duration.