Big is sometimes better
If you look at flying between two points on a long range operation then the cost per seat mile increases inversely to the aircraft available seats with current fleet types.
The aeroplane for long skinny routes would have to be a huge improvement over current types to keep down the cost per seat mile, which was what we saw with the B787 and explains why the airlines were so keen on it with Boeing projected 30% reduction in operating costs (if it fly's as advertised).