But shot from the same bow, the lighter arrow will go faster. Out to about 45 yds or so, the light arrow will drop less. Past that, the heavier arrow will retain a higher percentage of speed and drop less. I'm sure there comes a point of diminishing return, however. Slow the bow down enough, and the arrow will not go very far at all. I shot 305, and 375 gr arrows from the same bow. At 25 yds, same point of impact. At 35, the 375 gr hit about 2" lower. At 50 yds, nearly same point of impact. In a word, momentum. BTW, changing arrow weight only increases KE if the speed increases with the weight. The resulting speed loss, due to increased weight, causes the KE to be held in check.