The impact force they both experience is exactly the same.
This is according to the third Newton's Law: in every interaction between two objects, the objects act on each other with the forces that are equal in magnitude but opposite in direction.
For example, you attract the Earth with the force of the same magnitude as the force with which the Earth attracts you!
The reason this might seem counterintuitive is because we are used to judge the strength of impact (or any interaction) but the resultant change of the motion of the objects involved in the interaction. This change is more obvious for the objects with smaller mass. Smaller car will suffer more from the impact than the larger car. However, the force of the impact is the same.