As far as I can tell, the previous model used the same size battery box for the whole range. The original difference was the 51 LFP had 104 cells nom. 3.2v = 332.8v nom, the 64 NMC battery had the same 104 cells nom. 284.8v nom.
If they were truthful about the 51kwh and 64kwh battery capacity, the 51 would have used 104 x 3.2v x 153.25ah cells to = 51000wh and the 64 would have used 104 x 3.7v x 225ah cells to = 64000wh, so the 77 would have used 104 x 3.7v x 270ah cells to equal 77000wh ......
If they squeezed 130 LFP cells into the battery pack with the same 153ah capacity they could achieve the 64kwh capacity, but at 416v nom. that doesn't fit in the 400vdc architecture, so it must be a combination of 112 cells x 178ah cells with a nom. voltage of around 360vdc ........
The question remains, how did the squeeze the added capacity into the same size box?
T1 Terry