It's often assumed that the best stories are those which focus the most closely on their theme and/or plot. All things not central should be edited out. Every element must tend toward a single end. Even style ought to be disciplined, ought to be pruned of all nonesential verbiage.
Yet any number of writers have produced great work which defies the above guidelines. Melville, Tolkien, John Cowper Powys, Iain Sinclair — these writers produce fascinating, powerful work which refuses the centripetal urge. What I mean by that is that in Moby Dick or The Lord of the Rings there are any number of long passages, about whaling or the history of Middle-Earth, which seem to have nothing to do with what the book is ostensibly about. These are novels which resist the urge to cohere; which do not have a single centre of gravity giving unity to their text. A centre of gravity, in this context, might be a story or a conscious theme; it is some idea which is developed, and which gives obvious form to a novel.
Yet even books which resist the pressure of gravity still very clearly have a form, have some kind of coherency and structure which gives them a unity. The Lord of the Rings would not be what it is if the long passages on landscape or history were excised. Moby Dick would not be what it is without the long passages on whaling. A Glastonbury Romance would not be what it is without the shifting, multiple points-of-view which extend even to the cosmic forces of the universe. There is not necessarily a conscious attempt to pull all this material into a shape. It's just there. Part of the world the writer presents. Something the writer felt compelled to include, something which works in defiance of accepted critical strictures.
In fact, if you look at the underlying principle of said accepted critical strictures, you quickly realise that they don't make much sense. There was a Peanuts cartoon once where Linus bugged Lucy to tell him a story until she finally gave in and snapped out: "A man was born. He lived and died. The end." And that was the story. And the truth is this: If you really wanted to cut down a story as far as possible, there's really not much more to say.
So, let's consider the possibility that while it is true that what gives a story its shape is what you leave out, it is also true that what gives a story its meaning, its importance, is what you leave in.
Think of it like a graph. The horizontal axis is a measure of plot; the drive forward, the development of story or theme. The vertical axis, on the other hand, is a measure of the material the author brings to the story. The didactic material. The connections, the atmosphere, all the things which aren't logically relevant but which are artistically necessary.
Traditional critical thought, then, would suggest that a straight line across the horizontal axis is the ideal shape for a story. But I suspect that stories of that kind are at some level unsatisfying. Certainly a lot of cleverness in the construction of, say, a mainstream film story goes into disgusing necessary elements, making them seem like interesting side bits until the story reaches the point where they stand revealed as vital to the forward momentum of the plot. The result is something smooth, clever, and fundamentally empty. There's a lack of healthy eccentricity.
So if not a straight line, what? A straight line up the vertical axis would be at least as unsatisfying, in a narrative sense. Perhaps that's the line of nonfiction; of an encyclopedia. Narrative entirely forsaken, information finding its own order.
Likely the old image of a narrative arc is most apt. A story begins, slowly moves forward, accreting images and facts and background, then at a certain point begins to really rush toward its climax, with less extraneous material being thrown in. But the amount of vertical detail given earlier lends the arc a steepness, a velocity, as it races toward the finish.
Then again, perhaps some stories don't have a single line, a single equation, to them at all. Perhaps just as some cosmological theories suggest that the universe is closed, finite, and some sugest it's open and endless, so some stories are closed (horizonally-oriented) and some are open (vertically-oriented).
The point is mainly that there are a lot of different ways to think about stories and story structure; sometimes conventional wisdom is best ignored, opening oneself up to a potentially useful point of view.