I think lupo has some sort of a bellypan but not full smooth underside.
For lupo you need to also consider the crash safety because that is so small car, but as you mentioned there are cars out there which are under 700 kg. You just need to give up somethings you dont need, like the sound system and dampening materials
Iam willing to do that for my quest for under 2L @100 km/h. But most of us arent.