A central point in the article is that the distance the ball (or shuttlecock) travels seems to determine the size of the sports arena. The authors give the example of a game where a balloon is knocked over a net as in volleyball. They point out that the effective length of the arena is 3 meters because that is how far the balloon will travel no matter how hard it is struck. They note how the there is a clustering around the solid black line in Figure 2 below. In other words, the length of sports field is around the maximum length the ball will travel. They note that the two outlier games of Squash and Jai Alai (denoted with red squares) have walls that keep the ball in play.