Airline luggage and donkey pack boxes the same size

Not really sure if this has anything to do with anything, but have you noticed that the international size for aircraft cabin baggage is very close to the size of pack boxes for horses and donkeys? Pack boxes are side loads hung from a pack saddle. These are traditionally made from wood, cloth or leather. Modern ones are molded plastic. These are also about the size of the rugged cases used by military and emergency personnel.