Bavel no Hon

Both are short anime about books and danger that could be hiding inside them. While Wasurenagumo is a short movie, Bavel no Hon is a short special. 

Both are short, few minute long anime. Both feature part when main characters fight a whale that is trying to eat them. While Kujira is fantasy, Bavel no Hon is more for kids. 

Both are short anime with a bit surreal feeling and the idea of small world existing in a common place (on a top of someones head/in a book) and both have very similar animation done by the same person. Both were made by same director. 

Both are about a child/children that gets lost in a fantasy world. While Spirited Away is a movie, Bavel no Hon is short special. 

Tsuki no Waltz and Bavel no Hon both are shorts that go into a more imaginative point of view from what the children see.