Previous work examining the effect of vehicle mass has demonstrated the link with occupant injury severity. The principal factor has been related to Newtonian mechanics. This article analyses data from the U.K. Co-operative Crash Injury Study and identifies other factors associated with car size. The mass of the car is found to have a predominant effect on injury outcome in frontal collisions only where the effect is seen most in injuries to the head, face, and chest. Most fatal casualties in small cars occur when in collision with another car in front or side collisions while the key group for large cars is frontal collisions with roadside objects. There are several characteristics of small car occupants that differ from those in large cars including gender, age, and vehicle occupancy. New information in the analysis concerns the priorities in casualty reduction between small and large car occupants, and the article argues that vehicle design should take account of this variation to produce vehicles optimized for the complete range of crashes and car occupants.