Think cubically! A 6m device occupies 666 (say) --216 m^3 a 10m device occupies 10 10 6 (say) -- 600 m^3 The scale of volume means that you have to build a m

Think cubically! A 6m device occupies 666 (say) --216 m^3 a 10m device occupies 10 10 6 (say) -- 600 m^3 The scale of volume means that you have to build a much bigger facility to put it in (in order for the electronics to be kept dry and for people to be able to get around it to keep birds off it and things. But worse - the weight. Concrete is 2400kg m ^3 so the small device might weigh 518 tonnes, but the bigger device is 1440 tonnes, so moving parts of it round becomes 3 * harder, the floors have to be 3* stronger, the supply chain has to be 3* better. And then time - 3* scale, 3* engineering challenge -> many times more time to deliver, many more $$$ -> risk -> planning -> admin... the less capital at risk the less it's worth spending on avoiding the risk.. the less the overhead of the project is. FWIW ITER is a science experiment - it's designed to find out more about fusion and that data will be very valuable for future reactor designs.