Interesting question! I dont understand how any project file could get that large. I have seen project files of an amateur videographer approach 50 MB. It took 3-4 minutes to load and then finally got corrupted. Other rather simple projects of his were about 25 mb.
We have had discussions here with no conclusion. I suspect that editing style might cause it, specifically editing in the timeline rather than the Source window.
I just finished a wedding…4 sequences, 1 hour long, tons of cuts. Project file size is 1.7 mb. Dance recitals in excess of 2 hours, 2 camera shoots and seldom in excess of 3 mb.
New to CS3 is embedded titles into the project file which might make it a bit larger than 2.0.