Bed size is measured in feet. There’s never been a light-duty truck with more than an 8’ box, which is probably what the first one depicts. The last one might be a 5’ box but way more 6’ boxes get sold. So yah, they’re smaller, but they sure as hell aren’t half the size. Putting it terms of overall length is disengenuous at best.
I have an 8’ box because I want a toolbox and still be able to get a lot of undesirable stuff in the box besides. I wouldn’t have a shortbox.