They're light because of convective heat transfer. (Actually, "radiators" should be called "convectors", but that's another story).
A radiator is there to exchange heat from the coolant to the atmosphere (another contribution to global warming). Because we can't expose the coolant directly to the atmosphere, we need an intermediate step. Thus, the radiator. Hot Coolant -> cooler radiator -> cool air stream.
Anyway, think of it in terms of "inertia." If the radiator was "heavy," then once it heated up, it will stay hot, thereby reducing its ability to transfer coolant heat to the atmosphere. (The transfer of heat is proportional to the temperature difference between two bodies.) The amount of heat dissipated by a hot radiator is less than that of a cool radiator.
A heavy radiator will stay warm longer, thereby lessening its ability to cool in the airstream. A hot radiator will transfer less heat from the coolant than a cool radiator, thereby the coolant (and engine) hold their heat.
If the radiator is light, it holds less thermal energy, and once you hit the airstream, it cools more quickly than a heavier radiator, and therefore allows your coolant to cool more quickly because the hot coolant hits a cooler radiator.
This ability to quickly shed heat is important,especially with woods bikes. if you're working in tight woods, the coolant and radiator get hot. Hit the open air, and a light radiator sheds heat, and your engine temp drops.
I hate rainy saturdays. I spend too much time typing.
