One of the lesser-known skills of writing for print is the ability to write to length. Remember in school when your teacher assigned you a five-page paper? Yeah, it's sort of like that. If your first draft comes up short, you'll have to sit down and come up with some more information. If you have too much, you'll have to decide what to cut.
When writing a book, your length target has a bit of flexibility. You may have contracted to write a 500-page book, but your editor is unlikely to get upset if you turn in 499 pages or 501. But if you're writing for a magazine, your length requirements are much tighter. If you are asked to write a four-and-a-half page article, you'd better write a four-and-a-half-page article. You still have some room for fudging, say by adding an illustration, but the old schoolboy tricks of changing the margins or the font size won't work here. The magazine has its house style, and you don't get to say, "For my article, please use 11pt instead of 10pt."
If you're like me and have a fixed-position column, things are even less flexible. My column must be one page long, exactly. If I go over, there's no continued on page to overflow into; if I fall short, there's no advertising to pick up the slack. When I get proofs back from the editors, they will often have remarks like "This article is ten lines short. Please fix." (They don't often tell me my article is long, because editors are very good at cutting on their own!)
One thing that struck me when I visited the TechNet staff a few years ago was that many people expressed their appreciation that I hit my length (though not usually on the first try). I thought this came with the job of writing: You agreed to write n pages, so you'd better turn in n pages. Apparently, a lot of writers don't realize this. I'm told that when the editors tell a writer that an article is short and ask, "Can you add another ten lines?" they response they get back is sometimes a simple, "No, I don't think there's anything more to add."
Print periodicals have space requirements. When you're writing for a newspaper or magazine, you have to meet them. That's part of your job as a writer.
Bonus chatter: Recently, I've found that when the editors cut for space, they have at times failed to detect which of my sentences are "filler" and end up cutting the "good stuff". I've started including notes with my manuscript about which parts should be cut first.