I wrote an essay back in varsity that led to me co-authoring my one and only published journal article. The essay and article weren't that similar, with the article focused on socially constructed meanings given to nuclear weapons, while my essay had been about weapons of mass destruction in general, and had taken a more elementary look at what these weapons physically do and what this meant they had in common (or not).
A quick look at the standard legal definitions of WMD explicitly holds them to the level of destruction of a nuclear bomb, and yet biological and chemical weapons employed in reality just don't get that destructive. No single device built so far gets as destructive as a nuke (causing somewhere close to hundreds of thousands of casualties from one explosion in a population center, possibly more; the definition will vary with location and situation). That's why they're so scary. In comparison, individual chemical weapons have never caused comparable harm, and even sets of them used together have still fallen orders of magnitude short of nukes. Biological weapons remain mostly useless (compared with both nukes or chemical weapons), a dangerous concept to prepare for in future, rather than a practical reality today.
This doesn't diminish the destruction caused by smaller devices. It also doesn't diminish the cumulative destructive potential of large numbers of small devices (if we're not strictly talking about the effects of a single device, then any army, collectively, is automatically a WMD too). It just means the legal definition is very fuzzy or very stretched.
One possibility this fuzziness opens up is that some very large conventional explosives might legally be classed as WMD. They still won't reach the level of even very small nukes, but they can easily exceed the damage done by the chemical weapons that are already classed as WMD. That, to my non-lawyer mind, would seem to imply that large non-nuclear bombs ought to be considered WMD too (or that the definition needs a major rebuild).
These thoughts came back to me this week, as Syria appears to have made another chemical weapon attack, and the US has dropped its very largest non-nuclear bomb. I can already see the media (and thus the wider public too) getting mixed up about the nature of WMD, and what to do about them. This sort of confusion only makes it harder to discuss intelligent de-escalation or the hypocrisy of violent "peacekeeping". Whether the formal legal definition of weapons of mass destruction changes or not, it may be wise to push the term out of widespread use. It doesn't seem to serve a useful conversational function, and it invites a lot of misleading double meaning.