That's a good question. I wondered the same thing. The "wide" winged planes I've flown do seem like they have an especially slow stall speed. It seems the wider the wing the more lift. I think of it as free lift because your adding area without adding drag.

BUT, that doesn't explain why sailplanes use a huge wingspan narrow wing!? Given the above thought, it'd seem like my 2m Spirit would have a hugely wide wing but a moderate length for the most efficient lift (less drag). I dunno. I do know that the 2m Spirit has a crawling stall speed though.

Maybe I'll ask in the glider forum. I think there's an aerodynamic forum too.