What exactly makes a headphone "OPEN" or "WIDE"?

The K702 is ultrawide and so is the HD800s but when we look at teardowns, the K702 has like a plastic retainer around the driver which has a small annular opening around the driver covered by paper or something, and the HD800s is mostly open with direct mesh to the outside. Even the back of the driver of the K702 is not exposed to the world, just a piece of plastic in the way with a bass port. Here is my question, what do headphones do to be wide? The K702 is wider than the HD660s even when the HD660 is mostly mesh backed without any padding. So what is this openness?