Yes, that's probably it, Mark.
Wider harnas lines probably don't let any (or much less) wind "escape", while with narrower harnas lines you easier open up (although not necessarily) the sail in overpowererd conditions.
It remains strange though that a speed king like Antoine, uses much narrower harnas lines.
Some use almost zero width harnas lines. Take a look at the picture of the Isonic product presentation of SB's web site: http://www.star-board.com/2012/produ....php?id=isonic