Yes, a 600 f5.6 PF with a 107mm window will be shorter than the 2.4kg 800 f6.3 PF (127mm window). So arguably this 600 PF should weigh under 2 kg.
A 500 f4E PF needs a 125mm window, so also shorter and lighter than the 800 PF. A 1000 f8 is also a 125mm window but needs to be longer so heavier.
Read through the earlier threads with links to Nikon interviews etc, cited above. These have covered many of the design trade offs etc.
Nikon has probably examined how to scale down the 800 PF into a 500 f4E PF, or to scale up the 500 f5.6 PF to the 600 f5.6.... in fact there's a link above to the patent for a 600 f5.6 PF