The thickness used in aluminium radiators seems to be less than that used in copper radiators - I'm not sure why, but probably because of different fabrication methods. This probably accounts for the smaller size, remembering that heat flow depends on the distance the heat has to travel through the metal as well as the thermal conductivity of the metal.
But probably the difference in size is also a function of the structure of the core, which may be better for several reasons - different fabrication methods, the strength weight ratio of aluminium is much higher than copper, so tubes and fins, not having to support as much weight, can be thinner, and hence closer together, and the aluminium designs probably benefit more from computer aided design simply because they are newer designs.
John

