In a short story I can only show my rebellious fairy godmother character, Eileen, in one, maybe two situations (allowing for word count limits), which inevitably limits how much I can show of her real nature and capabilities. In a novel, I can expand the situations and the extent to which I can portray her. There is also room for sub-plots and minor characters. When these are portrayed well they add to a novel. One of my favourite books, Pride and Prejudice, is not about Mr Colllins as such but the novel has the room to show how Elizabeth handles his marriage proposal and that not every woman is as independent minded as she is. There simply wouldn't be the room in a short story to cover all this (at best it would be done skimpily). Besides it gives Jane Austen more space in which to show just what a repulsive character Mr Collins is (well he is to me anyway!).
Being totally lost in another world...
And again that world is larger in a novel. I love P.G. Wodehouse's Blandings stories and much as I admire his short stories, I feel as if I'm immersing myself properly in his world when I read the novels. When it comes to writing my books, I can put in those details which help bring my world(s) to life and which would be the first things I'd cut when editing a short story (as I wouldn't have the luxury of the room to leave them in).