DIAGNOSIS Yeah, this is a common misunderstanding. The size selector doesn't change the physical size of the cube mold. It changes the freeze time, which affects thickness. The difference between S and L is subtle—maybe 1-2mm thicker.
FIX To see the difference, you have to let the machine complete a full cycle on one setting, then switch and do another full cycle. Compare cubes side-by-side. They come from the same small, round molds, so they'll always be the same diameter. The "large" setting just lets more water into the mold before freezing, making a slightly taller, less hollow cube.
IF THAT DIDN'T WORK If there's literally zero difference, the button or control board might be faulty. Try a power reset (unplug for 2 minutes). If still no change, the timer circuit for that function is probably stuck.
HEADS UP This isn't a commercial machine that makes different shaped ice. The size setting is the most minor of adjustments. Don't expect restaurant-style large cubes from the small setting—you'll be disappointed.