It's kinda the same concept that was used with the Creedmoor. This new 6.8 Western actually has a little less case capacity than does the 270 WSM but the WSM is a bit longer of a case so when you stick a heavy for caliber high BC bullet in that longer case and use it in a short action where you're limited on overall loaded length you have to seat the bullet deeper down in that larger WSM case which eats up case capacity.
The 6.8 Western has less case capacity but because it's shorter when you use the heavies in a short action the useable case capacity is about the same as the WSM.
Put both in a medium action where OAL restrictions are lifted and that extra case capacity of the WSM will show its strength.
To me the 6.8 Western is kind of a "improved" RSAUM or 6.5 PRC and in short action its a good fit. It'll be interesting to see who all makes brass for it and how good it is.