It means that the pages are bigger in dimensions. I think it's a slightly outdated phrase though. Fifty years ago, when paper was cheap and there was no Web, you had two big distinctions between magazines: "Digest-sized," like the U.S. magazines "The Readers' Digest", "TV Guide," which were maybe 14 by 21 cm., and large-format, like "Look" and "Life" and "The Saturday Evening Post," which were maybe 30 by 35 cm. The second difference was between "the slicks," printed on shiny paper, and "the pulps," printed on newsprint. Fiction magazines, particularly genre fiction like mysteries, westerns, and science fiction, were usually pulp magazines in digest size. I'm not sure, but I think Der Spiegel is a large-format, slick magazine?