Up until around the time of World War II, most cartoonists got a full page for their Sunday strips. However, some syndicates forced some cartoonists to fill their space with two different strips, one using about three-fourths of the page and one using the remainder.
Faced with this situation, most newspapers in those days chose to brag about having twice as many comic strips as they would otherwise; a few newspapers, though, dropped the smaller strip and used the available space for something else.
That was, in effect, the first example of flexible Sunday comic panel arrangements. During and after World War II, when newsprint became more expensive and the size of Sunday comic strips shrank, syndicates started requiring cartoonists to draw their strips with panels that could be dropped if necessary.
Some daily strips have been made available with flexible panel arrangements, too. "Peanuts" is the most famous example of this; until 1988, Charles Schulz drew the daily strips as four square panels, which newspapers could choose to run horizontally, vertically, or in a square.