Radiator Size ????

Copper tube is more effective at drawing the heat out of the coolant to begin with.
Look at HVAC , copper tube, alloy fin for the standard version. Copper tube and fin for the more expensive. A lot of it just comes down to cost.

Here is a quote from a HVAC manufacturer .
"It’s really not a coincidence why HVAC coils use copper tubes and aluminum fins. Copper is great for heat transfer, and aluminum – while still very effective -is simply not as good. The first goal of any HVAC coil is to cool or heat. Heat transfer is always the first consideration. Cost is the second. Copper works well for the tubes, but would be prohibitive for the fins. You would need a compelling reason for the fins to be copper, and sometimes there are reasons to do just that. However, the vast majority of HVAC coils that you see are built with copper tubes and aluminum fins.. That combination offers the most effective heat transfer at the most efficient cost. "

Anyway....most go to an alloy radiator because they are cheap, not because they think it's a superior design , material or contains magic.