I think the premise is that they made the diesel nozzle bigger so that it wouldn't fit in a petrol filler neck because burning diesel in a petrol engine is far more catastrophic than vice versa - petrol in a diesel is a relatively easy fix. Burning diesel in a petrol engine gums up the plugs, valves and piston rings, as I recall it. You have to take the engine apart to clean off the residue and carbon build-up. In a diesel engine that's burned petrol it's more or less a case of draining the system, flushing through and then running some sort of turbo-nutter-bastard high strength Redex-like additive through the first tank of fresh diesel, or something. I might look it up in a minute if I can be arsed.Making one nozzle smaller than the other won't stop people putting the wrong fuel in as the smaller nozzle will still fit into the lager filler tube.
If they made them completely different shapes like oval, triangular or square then one would not fit into any other machine.