The answer for this change in the magnitude of momentum is the same for both because momentum is always conserved so both vehicles have the identical change. So for determining who has the greater change in kinetic energy, momentum (P) = mv so P^2 = m^2 v^2 P^2 / 2m = 1/2 m v^2 = energy So the weightier the mass the smaller the energy change for the same momentum change so in here, the car has a greater change in kinetic energy.
In order to understand this you need to remember that the momentum is divided equally in the collision, so the correct answer to this is the fact that their momentum will changes the same, because that is how energy is preserved, the thing that could be different is that they are affected different by that momentum, it would probably affect differently the small car and the tractor, it would probably have a lot of impact for the small car.
