Now that I think about, I wonder if the Principle Doctrines themselves were the "synoptic" overview of a longer and more detailed text, where he lays out the main points in the introductory material and then explicates each main point in more detail after. The surviving material by itself would make for a very short scroll--although it's possible that was the point.
I suppose the counterargument to that theory would by the Vatican Sayings, culled from many other works, where brevity was the whole point.