The issue with the A9 is not the size of the camera body.
It is the size of the frickin' lenses attached to the camera body.
I sold my Sony kit largely because the lenses (at the time) were pretty mediocre and because the lenses were totally out of proportion with respect to the bodies. What is the point of a tiny body and a lens which looks like it will tear off the lens mount?
This will always be an issue with FF mirrorless bodies unless lenses are specifically hobbled, like the Z mount 24-70 f4. Everyone is raving about how small it is. Yeah... stupid... it's been made f4 because the existing f2.8 is ginormously out of proportion on the Nikon mirrorless bodies and at f4 the whole lens can be a lot smaller.
The only manufacturers with small lenses in proportion to the body sizes are Panasonic, Olympus, Fuji and.... Leica (of course).
Sorry to be off topic - I actually quite like the specs of both the Nikon mirrorless and the A9. As I said elsewhere a Z500 would pair very nicely with the PF300 or PF500 Nikkor lenses.
LouisB