You are right I think, it's about impulse, force over time, surface area and how much energy can be transfered through the surface area through the underlying t

You are right I think, it's about impulse, force over time, surface area and how much energy can be transfered through the surface area through the underlying tissue and create a ripple basically. I suspect that at some point, the 'transfer function' of energy just maxes out and mass of the vehicle itself is pointless over that point, as it's all lethal.