Both the P and Z expansions will be just as big as WoL. They are also going to be unique in how their single player campaigns move forward the story line. To sum it all up, every race campaign is its own stand alone game.
To bring it into perspective, WoL, by itself, has more content than all three race campaigns of either sc or bw. Each expansion will be just as big. If you play eah campaign with no cheats, it will literaly take you days to complete all three. This isnt just adding a few extra maps in.
I will be purchasing all three and thoroughly enjoying each one.