thanks andretsin, thanks Ken
I am speaking about slalom boards that are planing, your are right Ken.
My own explanation was that a wider board, or probably just a wider tail, make the rider "rail the board" easier, thus helping the fin to counteract the side force. In this case rail shape and thickness is also important.
But I am asking because I am not so sure, and I feel this is not the only reason.
Maybe with a wider board/tail rider stance and position realtive to the sail are naturally more optimized for upwind performance?