Less mass means less momentum, so less force is required to slow it down, which means it can slow down faster in the time between noticing the pedestrian and colliding.

Higher hood means less visibility directly in front of the vehicle. It also means it's more likely to hit the centre of mass so the body takes the full force and falls on the ground the vehicle is moving towards, rather than lower so that the legs get pushed out and the body ends up falling on the hood.

On the flip side, they are more visible and generally louder, so pedestrians might be making fewer mistakes on their end.

The differences aren't about when they hit someone at a high enough speed any vehicle will likely kill them, it's about the thresholds between a harmless bump and a fatal injury.

And even if the driver is the main factor, that's all the more reason to increase the burden involved in driving them.