WMD definition
I agree the definition is not that precise. I have understood it to refer to weapons that are very small in proportion to their ability to create widespread destruction. So, while the firebombing of a city may create equivalent destruction to a nuclear warhead, one cannot firebomb a city with something you can carry in one airplane, or a truck.
Same for biological weapons, which are worse than nuclear weapons in this regard, especially those that are self-replicating in use.