As per Jay Allen’s comment, if you are using Movable Type 4, make sure you have a cronjob with this in it:
cd /path/to/mt; ./tools/run-periodic-tasks
(where /path/to/mt is where Movable Type is installed)
This will make sure Movable Type clears out its session data and so it won’t eat your database for lunch. It also does a whole load of other magical things, especially if you have plugins, but keeping your session data in check is one of them. I thought I had this in place but it turns out I didn’t – hence why I ended up with several hundred megabytes of session data.