Somatotype Shotgun is the situation that regardless of who calls shotgun in the vehicle to be used, someone automatically gets the shotgun due to their girth or height etc, for convenience.
Darnell: "Shotgun!"
Matt: "Unlucky Darnell, Paul gets Somatotype Shotgun, he weighs 16st 5lb and my rear suspension just can't take the load."
Darnell: "Fair one."