Truthfully I think there is very little correlation between pulling the string hard and volume. The character of the attack changes but I feel like every string reaches a threshold, which is pretty low, on how hard you have to pull for max volume. Like DC points out, I've found it it all about a big heavy relaxed arm. The weight does the rest... for volume. Then your angle of attack, the speed of the pull, the muscle force can help to shape the character of the attack.