Crash test regulations dictate that you're supposed to break a pedestrian's femur, not the knee, as chances of disability are lower (I kid you not, it's true).
The higher bonnet line reduces the chances of the head striking the windscreen directly. The shoulder of the pedestrian takes the hit first from the bonnet.
There's supposed to be a distance of deformation between the bonnet and the engine valve cover to absorb the impact.
All the factors above cause cars to look "fat" from the front.