[Brightcolours]

Tony Northrup has released a video on the new range of Canon's R series of lenses ....... I won't even bother linking it !!

he claims 11 inches length for the 600mm and 13 inches for the 800mm and a guesstimation  price of $1500 and $2,000 respectively ....

.........he claims the amazing compactness of length of the new Canon 600mm ............. so I measured it ........the Tamron G2 is 10 3/4 inches long inc. bayonet.......... of which 1/2 inch is bayonet inside the camera!! .... so 10 1/4 inches.

So for the G2 you get a 50% price saving ........ a light of a minimum of 1 1/3 rd stop increase in light and the zoom  of the G2!

Uhmmm....
The Tamron zoom lens EXTENDS a lot when going to 600mm. So if you want to get an idea what he means with "amazing comopactness", compare it with your Nikkor 600mm f4, or with the Tamron extended to 600mm. 
The Tamron is 267.7mm collapsed to 150mm, and 344.3mm extended to 600mm (Canon mount).
Your Nikkor AF-S 600mm f4 FL VR is 432mm long according to Nikon USA.
The Canon EF 600mm f4 L IS USM III is 448mm long.
11 inch is 279mm long.
Kinda in line with the pretty neat Nikkor AF-S 500mm f5.6 PF VR, which measures 237mm in length (compare that to the Nikkor 500mm f4 (387mm) and you will call that PF lens amazingly compact too).

I think if you are honest, you can understand that that is indeed amazingly compact for a 600mm prime. Whether you would/should/could prefer a lens like that over a 150-600mm zoom that is faster, is a different matter. I bet the new Canon will be lighter than the zooms, though. Not that hard for an f11 lens.

Your Tamron must be broken though, if half an inch (~1 1/4th centimeters) goes inside the camera... The mirror of your DSLR will not survive. ;-)