No doubt, the phrase is obvious and probably came into use in many airlines. But in the Atlas context I stand by my post. In the 1999/2000 timeframe the 767 and A300 were both considered for smaller payloads/increased frequency in South America, and "light twin" was a management term for the yet undecided airframe in this market. And in the context of the heavy freight market the 767/A300 are indeed "light".

But in the here and now I admitt that the term is used in the bars to needle the 76 crews, which is getting to be a bit old.