It has to do with having more surface area and since your weight is the same, it takes more effort to sink the rail because there is more board riding on top of the water. Think about doing a perfect dive versus a belly flop, there is more water resistance on the belly flop because there is more surface area in contact with the water.
In regards to your stance, that is all personal. I prefer all the way out, but all the way out is different on a 138 vs a 145. Stance shouldn't effect edging other than a bad stance limiting your body's ability to get in its preferred athletic position.
Last edited by stephan; 03-12-2013 at 12:01 PM.