In an ideal world, when one was piecing together a publication, everything would just flow together perfectly with just enough material to fill the space. Not too much; not too little.
This being the real world, such optimal conditions rarely occur.
Too Little Space
Sometimes you will just have one or two lines too much of text and can make it fit by tweaking the tiny details like leading.
Often though, the extra material is far too much and you simply have to cut something. It could be an entire article, a photo, or segments from several articles, but decisions will need to be made. These decisions will be based on issues like relevancy and timeliness, size, and appearance (photo-less pages tend not to inspire much interest for readers).
You may have heard of the inverted pyramid often used in media as a form to write stories. Although some have questioned its usage, one advantage is that it can aid in the cutting process as the editor simply needs to jump to the end of the article where the details are less important to the story and start chopping. This allows an editor who may not know much about the story, or have much time to review it, to quickly edit it down.
Too Much Space
What tends to be a trickier issue is when you do not have enough material to fill a spot. Leaving an inch of space blank just because your article ended short of the allotted space looks bad and is an inefficient usage of space.
Often extra space means digging into the press releases that came in to find some small tidbit that fits both the space and the surrounding content. (Putting in an article about flowers on the sports page just is not going to fly.)
Another option is to have little filler “ads,” often for the publication itself, or a local charity or upcoming city event the public might be interested in. Where I worked, we always kept a few of these sorts of ads on hand to stick into the publication wherever they were needed.
Too Much and Too Little
A common problem is to simultaneously have both too much and too little space all at once. This usually results in headaches.
On a Senior Living page there might be way too much text, while on a Real Estate page there might not be enough. To maximize what you can get in, you may just have to break down and do a quick adjustment of where your content goes—which includes ensuring everything adjusts properly, still makes sense, and nothing important gets lost in the transition.