Not trying to be mean, but the premise of your post is old school and out dated. Bigger does not always equate to slower. If done properly, bigger/leaner, more often than not will mean faster/more agility and quickness. Keep in mind, fast and quick are not synonymous, and quick is more important than fast for most position groups. I’d be more concerned with body composition than overall weight. Two guys 6’3” 220 one has 12% body fat, the other has 7%, the 7% guy is going to be faster and quicker, despite the fact that he weighs the same.
I have a feeling Duval is going for a combination of strength, speed, and stamina. I have a feeling weight room days are a combination of power lifting for strength and dynamic, explosive lifts for speed. I.E. 6-7 sets of heavy back squats, followed by another 6-7 sets of heavy deadlifts, then follow with body weight lungs super set with box jumps. Polish it off with a few sets of hang cleans, and you just combined strength with speed. On off days I’d Imagine they are doing sprints, with explosive side to side movements. All of this would likely lead to less fat, more muscle (which could mean a heavier weight, muscle is more dense than fat) and enhanced agility. The weight these guys are gaining is designed to make them faster/quicker, add power to push opponents around, and develop stamina to outlast opponents late in the game.