"Wayne -- any descriptions on the size of Merion's green? Wider than deeper, or the other way around? Do the internal contours effectively create a smaller target area for certain pin placements?"
The 13th at Merion East is small, approximately 3200sf. As can be seen in the drawing below, the green is about 19 yards deep and 20 yards wide. With the green firm, the slope off the fronting bunker tends to propel shots into the rear bunkers, making for a difficult recovery. However, almost all of the green space is pinnable with a number of interesting pin positions.
With small greens with interplays of slopes (I like that a lot better than a horseshoe or other unnatural-looking features) that extend to fall-offs and wrap behind bunkers there is a challenge to all golfers. The better players will try to attack pins on the peripheries and could get into trouble if they don't pull off a good shot. High handicappers will try for the center of the green (if they play smart) and their own variance in execution could get them into trouble. A safe play to the middle of greens, especially on short par 3s and approaches does not seem to be the decisions that many better players make.