I get the feeling that on the flats the lower weight riders also have the same watt/kg advantage as uphill. On flat road, in real life if I (105kg) push 350 watts for a time I easily drop 70kg riders putting 230 watts on the pedals. In zwift they go as fast or even faster.
I agree on the watt per kg uphill. But flat and down Hillegom are not realistic in my opinion.