looks neat and I am in no way any kind of expert; I wonder why there is an external counterweight? It seems to me that the rotating 'piston' has hollow chamber

looks neat and I am in no way any kind of expert; I wonder why there is an external counterweight? It seems to me that the rotating 'piston' has hollow chambers in one side only to allow passage of gases and therefore needs a counterweight to balance it; is there a reason they cannot have some hollows in the other side but not have them connected to the burn chamber so that the piston is balanced by design, and the external weight not required?