The deadly forehand.
At 6'6", Juan Martin Del Potro needs quite a knee bend to get a two-handed backhand ready for a low ball. With a one-handed slice, he could expend less energy, but without topspin, he couldn't hit the ball as hard and keep it in the court. In this photo, the position of Del Potro's racquet, well below the ball, indicates that he does intend to use topspin.
Juan Martin is using the most common two-handed grip, with his right hand in a Continental position and his left in an Eastern forehand position. His stance is slightly closed, but not enough to interfere with his swing. Generally, the bigger and stronger a player is, the less backswing he needs to generate power, and a more compact backswing has the advantage of simplifying timing and certainty of contact.