Originally Posted by Buda
Hardly anything is printed on an A4 sheet these days.
Hmmm, it never even occurred to me that the original poster might be thinking that magazines were printed one page at a time.
As Buda said, most things are printed on much larger paper sheets or, even, long rolls of paper when the printing is done on a web press. With a magazine, for example, many pages are printed at the same time on a large sheet or roll of paper. That paper is subsequently folded up and trimmed to size, with all the pages automatically being folded into the correct position.
Paper and presses come in many different sizes. With a magazine (or any other printed material) the printer can work with the publisher to optimize the size of the magazine to take full advantage of the presses and paper to get the most for the least and with as little waste as possible.
A huge-run, high-printing-quality magazine, like National Geographic, might possibly even order custom paper directly from a paper mill.