The difference in mass between a pedestrian and any vehicle is so great that at the moment of collision, the pedestrian's velocity becomes that of the car, and

The difference in mass between a pedestrian and any vehicle is so great that at the moment of collision, the pedestrian's velocity becomes that of the car, and the car's velocity is practically unchanged. (That is, energy transferred is a function of vehicle's velocity and pedestrian's mass.) Mass of the vehicle only practically comes into play for stopping distance, and as a proxy for its frontal cross-section.