An SUV hitting a pedestrian leads to bigger harm as the impact is higher up on the body. An SUV hitting a car leads to more injuries for the people in the car, as the weight of the SUV easily crushes the car windows and the frame around them.
I wonder if the weight is like the rocket fuel problem for larger vehicles. Famously the battery pack for the electric hummer weighs more than an entire Civic.
That really doesn't take into consideration the negative sum nature of car safety that your sibling comment goes into. The issue is that safety requirements for vehicles are largely about occupants. They don't include sightlines, maximum heights, maximum weights, etc. (things that would make other people safer) thus the incentive is to build tanks.
Parents certainly don't need a huge SUV to fit their kids in but, because there are other SUVs on the road, they do want it for safety. Put the right incentives into your safety regulation and cars will shrink.
SUVs make collisions more common and more deadly. Anyone who doesn't know these facts is so ignorant about road safety they have no business driving. Anyone who knows these facts and buys a giant "car" anyway is so selfish and reckless they have no business driving.