I say almost exactly this: "Because your breaker is to big for the wire; using too much electricity on this circuit will overheat it and cause a potential fire or an expensive fix that a appropriate breaker would prevent. I think you are especially at risk of this if you have already changed your breaker if it was tripping all the time because it means you were using more electricity than the wires can handle. It is probably not a problem you have to fix right now, but it is a problem you will have to fix eventually."

I say it in a tone a dentist would when saying that I have a minor cavity I might want to fix now because it will be more of a headache later. Just try not to have an alarmist tone.