In the last few years a number of additions to the technical literature on relationships between car size or mass and occupant risk of fatality or injury have appeared. This new information is reviewed, synthesized and used as the basis for additional calculations aimed at better identifying causal factors. It is concluded that if a car crashes head-on into a 12,000 kg truck, the car driver is 36% more likely to be killed in a 900 kg car than in an 1,800 kg car solely as a result of differing Newtonian kinematics. Five studies from two countries consistently support that when cars of similar mass crash head-on into each other, driver risk is inversely related to the common car mass. Size is the dominant causative factor in this relationship, and in the higher rollover risk in lighter cars. Mass and size are causal factors in single-car nonrollover crashes. Mass exercises a dominant causal effect on car driver risk in crashes between vehicles whose masses differ by more than about 10%. All cars becoming lighter/smaller would have only a small effect (most likely an increase) on net driver risk in two-car crashes, such crashes accounting for 22% of car-occupant fatalities. As 70% of car-occupant deaths occur in crashes involving only one car, and lighter/smaller cars increase driver risk in all of these, a smaller/lighter fleet leads to increased casualties regardless of any interactive effects in the two-car crash component. Because mass is a dominant causal factor in crashes that account for over 50% of car- occupant fatalities, mass reductions (even if size remained unchanged) would lead to casualty increases. Any measure that reduces the mass of cars, even if car size remains unchanged, will increase car-occupant fatalities.