Really? The A9 competes against the D5 and is hugely smaller and lighter and coupled with the much smaller and lighter 400 2.8 makes a sports package that saves an awful lot of weight and bulk. You don't feel that contributes to the mirrorless compactness? Really?
Add the A73, which is a better camera than the Z6, coupled with lenses like the Sony 12-24, 35 2.8, 55 1.8, 85 1.8 and the Batis 18, 25, 85 and 135...and you don't feel the Sony system has high quality light lenses to form a nice compact system? Hmmmm...
Yes, the a9 has all these qualities, but it isn't any more compact than the other a7 bodies. The Sony 400mm f2.8 is a great achievement in terms of weight compared to the previous iteration from canon (now in line with MkIII) and Nikon (4 years old), but it isn't more compact.
Sony: 158.1 x 359 mm
Nikon: 159.5 x 358 mm
The lens is so much larger than than the body that you'll need similar packs sizes for these 400mm lenses, regardless of whether they are mounted on an a9 or D5. As a D5 + 400mm f2.8 E FL owner for years now, I believe that the D5 has the right body size btw for these kind of lenses (and I hope that the Z8/Z9 matching them will be larger than the Z7), but this isn't the point of this discussion.
So I stand by my point that the compactness of the a9 has not been a relevant factor that has driven a significant number of photographers towards the Sony mirrorless system. The a7 did initially.
Yes, there are compact lenses in the Sony line-up, but they are overall of average quality compared to their G Master counterparts and in absolute terms.
Just compare the computed MTBF of the Nikon 24-70 f4, 35mm f1.8 and 50mm f1.8 to their Sony counterparts and you'll understand what I am talking about.
It doesn't take any fanboysim to see that Nikon has chosen a different route than Sony from that standpoint. Sony has focused on high-end G Master glass with very high level of image quality, but they are not compact at all anymore.
You don't need to believe me, just check the Sony line-up and tell my which G Master lens is compact?
Do the same with nikon S line (that I am sure you'll acknowledge is the equivalent of G Master) and 2/3 of them are compact designs.
Cheers,
Bernard