I believe that you are correct in your calculations for number of messages. However, remember that all users messages are stored in those 26 message databases. If you have 1000 users that would be 38 users per message database. If each user had 30,000 items then it would reach the 2GB limit. It all depends on the size of the Groupwise system. If you increased to 2000 users that would be 76 users per message database and they would need to have 15000 items each to fill it up.
Your Groupwise system may not be near this, but I have seen some large systems that had a lot of users and a lot of e-mail because they did not have any policies in place to limit or reduce the amount of e-mail stored. They did implement some changes because the Groupwise Maintenance would take a long time to run.