The rear motor doesn't take up much space, essentially built into the rear diff. The NX battery pack is made up of 204 cells, the IS uses 192 cells.
The issue being discussed here is that the NX kicks in the petrol engine much sooner than the IS, this isn't down to the battery capacity or the power that can be delivered by the electric motor - it just seems to be the way the system has been programmed to operate.