A pedestrian being hit by a small car or big car is likely ruined either way.
Vehicle size actually has a huge effect on the severity of vehicle-pedestrian collisions.
I find that full-size SUVs and pickup trucks pose a particular danger for pedestrians. A pedestrian hit by a full-size SUV is twice as likely to die than a pedestrian hit by a car under similar circumstances, while being hit by a pickup truck rather than a car increases the death probability by 68%. I find that high-front-end vehicle designs are particularly culpable for the higher pedestrian death rate attributable to large vehicles. A 10 cm increase in the front-end height of a vehicle increases the risk of pedestrian death by 22%.