Hi Spiros,
the Jacob&Co "Quentin" makes use of 7 barrels to achieve the same power reserve in a case of 56 x 47 x 21.5mm in size; that´s for sure not a small watch by any aspect, but the linear arrangement of barrels, lined up horizontally, may account for the sheer dimensions to some degree.
The new Panerai 10-days makes use of several barrels as well, which are "stacked" vertically pair-wise; that seems to be another approach.
Nonetheless, i´d agree in that using two barrels only require each to be likewise huge, which naturally limits the diameter of the movement to some degree.
Warm regards,
Peter