Impact is all determined by physics. In an impact with a stationary barrier, low mass is better. This requires less energy to dissipate to completely stop the car. This is a much more likely type of accident.
In a head on collision, obviously the vehicle with more mass will come out with less damage. This is especially true when you figure in the collision occurring at potentially a much higher location than the vehicle is designed for. However, much less likely to be in this type of accident, or to survive this type of accident in any vehicle.