I suppose that she could have a backup knife with a matching pommel, but that would require a lot of offscreen weapon-switching while hanging by one arm between weapons in invisible scabbards. It's not impossible, but it doesn't seem more obvious to me than a resizable weapon.
Well she can probably resize her sword because it is magical. (I don't know the page number, but Parson stated that enemies got to magic items dropped by Jillian.) Despite that the side view of the dagger/sword doesn't seem to look like her normal sword would. Plus when Stanley resized the book everything about it changed. Mabey Jillian had a hidden shrunk dagger.

On a unrelated note Jillian's eyes look grey again. I wonder why her eyes were blue in the thinkagram.