I don't think there can be a defined shape. Form certainly follows function when it comes to air pistols, the intended purpose, power plant and ergonomics put into a relatively small machine are always going to define how it looks.
Any styling is going to be secondary to those, hence why the 1911 "styled" HW45 looks nothing like a 1911. But I would agree that the action sat between trigger and barrel is the classic style be they Co2, springer, SSP, PCP or rubber band powered.