One important feature of a novel is that there are lots of characters and subplots. Because the novel is longer than a short story, the author has time to develop ideas in more depth and tie together the stories of several characters into one, ultimately cohesive, unit. The richness of the experience of a novel is to see how all the parts will unite to create the whole. It is theatre because the language of the novel creates the scene and action in the reader's mind, but the experience can set aside and rejoined as often as the reader needs to.
Novels have characters and settings, and the author is telling a story with those characters. What makes a novel pocket theater is that it usually has a complex and interesting plot. You can put it in your pocket and take it with you, because the theater is in your head. The novelist's job is to set the stage and describe the action.