This is why the risk is greater if you are sitting in a small car. The law of mechanical work tells us that the transferred energy is directly proportional to the force and to the deformation of the object caused by the force during each moment of deformation. A part of the energy is transformed into other forms, e.g. noise and heat in buckled structures in the two cars. Another part is transformed into kinetic energy of the impacted car. The acceleration of the impacted car is related to the transferred kinetic energy and to the duration of the impact.
Metal sheet is more easily deformed than a metal girder.
For a specific impact velocity, the impacted car will accelerate less if it is only its metal sheet that is struck than it will if the impact is against the bumper, which is fixed to the girders. In many cases, the bumpers or the girders will not even be damaged. In the latter case, the main part of the kinetic energy of the car behind will be transferred to the kinetic energy of the car in front - the impact is almost elastic. For the same impact velocity in the first case, when only the metal sheet is deformed, only the kinetic energy not absorbed by the compliant structures of the cars will be transferred. In that case, the impact is mainly inelastic or plastic.