According to the FAA's definitions in 14 CFR 1.1, a Large aircraft is an "aircraft of more than 12,500 pounds, maximum certificated takeoff weight." I have heard this threshold was set based on the DC-3, but that makes no sense to me. Also, there doesn't seem to be any common-sense/international connection to kilograms (i.e. 12,500 pounds is roughly 5670 kg, not a nicely rounded-off 5700 kg).
What is the origin of 12,500 pounds being the cutoff between Large and Small aircraft?