From the game book:
Bob: The wereraven example reminds me, William—shapeshifters of the sort that can change their mass are a very rare breed. If they shift into something smaller, they have to store part of their intellect and mass in the Nevernever, so they can retrieve it when they change back. If they shift into something bigger, they can get the extra mass by way of ectoplasm, so in that respect growing is easier than shrinking.
Billy: Yeah, I thought I remembered seeing something about that. Still, we figured it’ d be more fun if we just allowed folks to go for bigger or smaller forms if they could find the points for it. And the whole thing about storing parts of themselves in the Nevernever is rife with story hooks.