Brittney Lee, co-director of Frozen, shared on her blog images and her perspective on the costume design in the film.  She addresses the need to have multiple costumes for the main characters as they age and develop throughout the film.  A fun additional tidbit about Brittney is listed in her profile.  She lists her favorite movies as The Little Mermaid, Lilo and Stitch, The Incredibles, Eternal Sunshine of the Spotless Mind, Big Fish.