The forces generated in collisions are determined by the impulse imparted to the car and the truck.

Impulse is the average force generated on an object and is equal to the change of an object's momentum divided by the time it takes to change. `F = (Deltap)/(Deltat)`

The collision time will be the same for both the truck and the car. Because the truck's mass is significantly larger than the car's we would expect the change in the truck's momentum to be significantly smaller than that of the car, therefore the car will have a greater change in momentum. The greater change in momentum will generate a greater force.