The physical dimensions are the same..as to your other questions, I don't know. More turns on the primary , less on the secondary. With the secondary voltage being the same in both transformers, there would be less turns on the secondary winding on the 240 primary than the 120 primary. So...would it actually couple better?? Don't think so. Personally I think it's the ability of the higher voltage primary to supply the secondary with voltage and current with less sag on the primary. Transformers are funny things....