A small correction: The difference in strength between galvanized G40 and G70 is not anywhere near a factor of 2, it is actually more like 20%. G70 chain is, however, roughly 2X as strong as G30 ("BBB"), so that might be where that number comes from.
According to the data I have, the specified breaking strength for G40 is 16,200 lbs, while for G70 it is 19,800. Working Load Limits (WLL) is usually specified for these kinds of alloy chains as 1/3 the breaking strength.
Annapolis, MD, USA