lights + engine size
Volvo lights are permantley on when the key is turned to power-up There aren't that many situations where you would sit with all the electricity off but the lights turned on, so an alarm is hardly necessary; in contrast to my volvo. Likewise on my saab the lights are on- it's possible to overide this to be off or sidelights only when the engine is off; but this does have an alarm.
I'm also intrigued that the engine size plays such a part in the economy, surely if driven in the same way, a modern diesel in the same weight of vehicle consumes the same fuel no matter the engine size, isn't it the engine management system that is making the difference here?