My guess would be because the larger truck loses kinetic energy at a slower rate do to the momentum gained. a heavier object may take longer to start, but it's also harder to stop. if the two masses collide, then my guess is the heavier truck would push on the lighter truck with a greater force than what's being pushed onto the heavier truck there by stopping the lighter car with energy to spare. So where the less massive object loses all of its kinetic energy, the more massive object has left over kinetic energy to spare. assuming that the lighter object is traveling at a faster pace then the heavy object to equalize the difference in kinetic energy due to difference in mass, then this would cause the lighter object to lose all of its kinetic energy (lets say 200 joules) where as the heavier object would lose most of its kinetic energy but not all of it (like 190 joules). I hope this helps, and I could be wrong. this is simply the educated guess of a 17 year old who is trying to go into a physics major but has limited high school options so he can't really start until I get into college.
