It’s the pace of velocity change...in a collision all the energy is transferred to the low mass vehicle, which will then change velocity very quickly - much quicker than a larger car.
This principle was demonstrated quite nicely last night on Daily Planet with the drop of three stacked balls… a basketball, a volleyball and a tennis ball. The video isn’t posted yet but here is a similar experiment by some kids who just used a basketball and a tennis ball for the experiment. https://www.youtube.com/watch?v=hD1zlq0Rg98