They are not bounded by space, time or set participants. They can be played by one or many, in physical or virtual space.
As far as I can tell, they are the most inclusive area of gaming, as they include practically everything. Their one condition is a disruption of the 'magic circle', meaning if concentration on anything but the game is lost or ruptured, it becomes pervasive. Someone (else) moves one of the chess pieces. You think about a move while you're at work. It's pretty expansive.
As big as big games are, they are not pervasive by nature. It might happen that at times in play they move into pervasive territory, but they can (and most often do) require that players know they're players, spaces are defined (if at times enormous) and they have clear starting points and fairly clear end points (or a goal that can be reached in a reasonable time).
Funny how digging through stuff makes the big seem small.