[Rury]

It is kind of wrong. What matters is the force impacted upon the person (their acceleration), not momentum. Basically, change in velocity over time is what really matters, but because momentum is conserved, it influences what the total change in velocity is.

[FieryTransition]

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.