Well actually I'd say that the emails each go for at least 2 minutes with an average of about 3 minutes. Some are really long (Theme Park?) and go for 5.
So that's about 300 minutes which is one DVD (at least), then an hour of extra stuff which would make the 2nd DVD. Then I guess they also laid out the commentary which is another 2 audio tracks which take up a fair swag of space.
So I can see two discs being filled quite easily with whats just listed there, and they could easily make the quality bigger/better so it'd space into three quite easily.
Don't see how you'd do 100 emails + audio tracks and hour of extras on one DVD.