High volume depletes more glycogen, and depleting glycogen results in more GLUT4 translocation, which results in better insulin sensitivity, which means better nutrient partitioning. So the nutrients you ingest go to your muscles instead of your fat. The reason I use finishers is because achieving high volume with heavy weight is very difficult. I enjoy the strength adaptations that come with training in the low rep range, but I also enjoy the metabolic advantages of higher volume training. That