Off the top of my head, airbags, anti-intrusion beams, ABS, pedestrian protection (the hood has to be higher, which makes the whole car taller), rollover protec

Off the top of my head, airbags, anti-intrusion beams, ABS, pedestrian protection (the hood has to be higher, which makes the whole car taller), rollover protection (the pillars are much thicker to prevent the roof from collapsing). They're sensible things, but each adds weight. Most of them scale sub-linearly with vehicle size, so the burden literally weighs heavier on small cars.