A big factor is the height of the hood. When a smaller car hits a pedestrian, the hood is usually at or slightly under the pedestrian's center of mass, so the

A big factor is the height of the hood. When a smaller car hits a pedestrian, the hood is usually at or slightly under the pedestrian's center of mass, so the person will often roll up onto the hood. In a bigger car the hood is substantially above the center of mass so that a person cannot roll up --- they end up being forced under the car, which is much more dangerous.