Nikon Z 6 or D850?
They’re both full-frame (24×36) sensors, but they are not at all the same dimensions physically. An example
Here are two 24×36 format cameras, the Z 6 with a 24mp sensor, the D850 with a 45mp sensor. The lenses are ‘almost’ identical – the smaller one is an f/4 which explains, to some extent, the smaller size, and weight.
This image gives a slightly better idea of the overall difference in size, both of the camera bodies and the lenses.