I wouldn't call the IBM 709 a bigger version of the 705. The scientific machines (701/704/709) and the business machines (702/705) are really entirely different

I wouldn't call the IBM 709 a bigger version of the 705. The scientific machines (701/704/709) and the business machines (702/705) are really entirely different systems: completely different architecture, console, word size, instruction set, physical construction, and even basic internal circuitry and voltages. You're better off thinking of them as unrelated systems that marketing tried to turn into a coherent product line (see also Intel 4004 and 8008).