The correct term is
Datsun 1000 are the lightest mass produced front engined rear wheel drive cars in the world.
If you look for handmade cars like lotus 7 or Skaterhams then you are looking at a 2 seater in limited numbers.
To compare a fiat 500 then compare that to the Flying feather.

425kg vs fiat porky 499kg
or even a subaru 360 or suzuki fronte 360 the fiat had no chance with these rear engined samurais!
Datsun 1000 is the Datsun Special K
Like the Special K cereal its light rice from the 60s
Still the lightest rear wheel drive front engined mass produced car ever made to seat 4 passengers and make 100mph.
On an interesting note imagine a K ute in the following guise:
Engineered into tray it could lose another 50kg without the rear door and panel metal that surrounds it.
615kg minus 50 = 545 kg
then minus window mechanisms,
bumpers, tar,
Composite carbon leaf springs all round.
lexan glass sliding glass on the doors
Lexan rear window,
FRP bonnet,
doors, guards,
alloy radiator
and a simple
200hp turbo suzuki g13bb
(30kg lighter engine than an A)
Suzuki sierra box
youd have close to a 480-490kg tray ute
that competes and most likely kills almost anything
for a fraction of the price of other 2 seater specialised
race vehicles.
Take it to another extreme and remove the roof and
have rollcage made to reinforce the cabin area with
no doors and no windscreen just little lotus 7 mini
screens and youd have a crazy little mutha.