Oh, that’s much nicer than mine! I don’t create many Venn diagrams, and I couldn’t think of a way to make it work with circles, because I always assumed I’d have to include the intersection. This feels simpler to me. And I didn’t like my initial Tiddler/Shadow/System and replaced them with Plain Tiddler/Shadow Tiddler/System Tiddler rather than your much simpler Plain/Shadow/System.
I think you made the right call about the sizes. Plain is what we generally deal with most. Although I swear I spend more time in Plain/System tweaks than in just Plain. Maybe that’s just me. (And @CodaCoder 
.)