Type your cut contents here.
Let's talk production/marketing first. Consider a magazine. It has the same number of pages every month for a reason (an economic reason) and everything in it must fit on those pages--ads, nonfiction, fiction. The nonfiction writers have strict word limits. The fiction writers' work must be shoehorned in--and let's say there's space for 40,000 words of fiction. That's eight 5000 word stories, or four 5000 word stories and two 10,000 word stories, or four 10,000 word stories...etc. When that editor says "I can use your 6000 word story only if you can cut it to 5200 words..." he means exactly that. My first fiction sale came after four rejections from an anthology editor who said "At this point, I have room for only a 1500 word humorous story." I wrote a 2300 word humorous story and--understanding the editor was serious--cut it to 1497.
Books come in certain standard sizes (thicknesses, when you look at mass-market.) Publishers know exactly how many books of a given thickness will fit on a grocery-store/airport wire rack. Fewer fat books fit there than skinny ones. Fewer books on the rack means fewer sales, because those racks aren't restocked several times a day. The fatter the book the more paper (and larger cover) it needs; if 24 of a given length fit in a standard box size, fewer of the thicker books fit--and they leave gaps that need to be stuffed. It takes the editor and copy-editor and production staff more time to wade through that longer book. There's a sound economic reason to limit the size of these books. Yes, some readers like fat books. And a good fat book is better than a good skinny book because you get more goodness. But it's a risk....in a business already very risky. So editors may insist on length limits (top and bottom) as a way of controlling publication and distribution costs.
Story reasons also operate. Good writers, when editing their work, cut constantly. You know the traditional story of Michelangelo and carving statues--it's easy, he said: just cut away everything that isn't the statue. Well, it's that way with words, too, but somewhat harder to see. We are not gods, with perfect creative powers. We leave bits of wordage on our work that isn't the work...not the work we want it to be. Especially when first-drafting (sometimes even later) and especially (for me) when writing fast, what comes off my fingers is not pure gold--there's a lot of gunk in it. I don't much care, first-drafting--that's the stage at which the whole story comes out...what I hope is a lump of diamond or gold, whatever it looks like right then.
What kinds of things do we cut to improve the story? Obvious extraneous chunks...notes to the writer, trial runs at a difficult scene, blind alleys of plot that ended up going nowhere. For instance, in my first novel, there was a long passage in which my protag had an adventure that not only led nowhere but conflicted with another adventure later on, one that had strong resonance and was tightly bound to the whole story arc. I found that one. My editor found another--a beautiful chapter that, nonetheless, could be cut in its entirety without affecting plot or characterization at all. The structure of the story needs to be cleaned of all the clutter that first-drafting produces, so readers can enjoy the shape. When you run into "infodump" in someone's book...when you suffer because of the writer's personal passions or research...when a mystery or adventure story is interrupted with a lecture on psychology or politics or history...it should have been cut.
The prose itself may need (usually does need) tightening. Most of us are sloppy speakers and writers; we're surrounded by lazy, sloppy, wasteful language, language that does not say what it means the first time through, and thus takes more words to convey meaning--meaning that's obscured by the extra words. Most of us learned to pad our writing in school, where teachers may have pushed us to use adjectives and adverbs to make our writing more colorful, or to reach a minimum word length. Many of us depend on these words and never develop the vocabulary of nouns and verbs that enable vigorous writing. Editing for clarity may require cutting, change, or addition.
So if I find that I've written "...walked slowly..." I will consider which "walking slowly" verb might replace it: did my character saunter, stroll, or wander? Or something else? The right verb can suggest the character's mood and physical condition as well as the relative speed of the walk. Confident, even over-confident, characters saunter. Drunk ones stagger or stumble or weave. Angry or slightly disabled ones may stump. Most "--ly" adverbs suggest a weak, too-general verb.
If I find stack of adjectives desperate to decorate too general a noun, the same fix applies. That "very big, impressive house" may be a mansion--and if so, away go "very big, impressive." "She had shiny, long, straight black hair that hung down her back" could be "Her hair was a shining wave down her back." (Don't get too creative with this: I once judged a writing contest in which someone's description went from lush to ridiculous with a woman's hair...)
"There" often presages a sloppy, loose sentence, and always when treated as the subject, not the location: "There were five men in the room" says no more than "Five men were in the room," but adds a word. "There, over the mountains, loomed a great cloud of smoke and ash" doesn't need "there" for location--"Over the mountains loomed a great cloud..." or "A great cloud loomed over the mountains" gets rid of "there."
Complicated sentences are fine until they tangle--and most of us are not taught how to write untangled ones. (For a primer, read Ruskin. Matthew Arnold isn't bad, but Ruskin was an artist with the long sentence that reads perfectly.) Every long sentence should be read aloud, with extra attention given to any grammatical reversals, the antecedents of all pronouns, and the need for a) the length, b) the reversals, and c) phrases. Almost every long sentence begs to be pruned of exuberance--not made short and choppy, but perfect in its own form of length.
So cleaning a stretch of story nearly always shortens the part already written, even if the writer finds gaps to fill (motivations unclear, transitions too rough, etc.) This cleaning/cutting/editing improves the work, if the writer has some skill. The ideal is to cut off all the fat, but leave all the flavor...to cut off the non-sparkly parts of the diamond but leave the best, unflawed sparkly bits in clean shiny facets...to polish away all the rough edges of the machine, so it operates flawlessly, smoothly, as it should.
There are more scalpels in the writer's toolbox, more techniques for unobtrusive pruning-to-fit (some of them I've talked about recently in some posts on editing here. ) Learning to cut, and cut well, is one of the skills a writer--any writer--needs to develop.