It's a safe assumption that safety requirements were a big driver of increased size and weight. And some of this factors have a snowball effect. Some requirements make the car bigger and heavier so it needs bigger engine, wider tires, bigger fuel tank, then it's even bigger and heavier.

Add all the luxury elements that attract customers and you have a lot more extra weight. Small cars are generally seen as entry level, unsafe, so car manufactures are more than willing to provide bigger, fancier models. It's not just the size that shot up, prices did too in order to cover all the extras.

People are also bigger and heavier than they used to so extra space and carrying capacity are more than welcome.