Hey Nat, and all who are experiencing this problem,
I too found that my Premiere CS5 project file sizes skyrocketed for no reason, so I did a little googling and found this topic. After reading your response that Adobe provided saying it had to do with the rendered files accidentally being saved within the project, an idea struck me.
I went to Sequence/Delete Render files, then saved my project as a copy. Lo and behold, it went back down to 6mb! Yes, you’ll lose all your preview files, but they can be easily generated back by some overnight rendering, and in your situation it’s at least a temporary workaround so you can save your project. Hope it works for you as well!
Nathan