You're kind of stretching the truth here. There really isn't a limit as to how much explosive material can be contained in a stationary "bomb" before it's considered a WMD because the definition itself takes in account intention of use.

The Wikipedia article cites the FBI's definition of a WMD, which includes rockets that contain more than 4 ounces of propellant. For comparison, an Estes D size rocket engine is less than a single ounce (~0.8oz). It also states that any rocket with an incendiary charge of more than one quarter ounce is a WMD, but if you actually read the statute and not Wikipedia's summary you'll read that the charge has to be a known explosive substance like C4 or ANFO, not something silly like Draino and aluminum foil or dry ice in a plastic bottle, unless there is a clear intention of causing harm, injury or death. Here's a video of 10 grams of C4, which is about a third of an ounce: https://www.youtube.com/watch?v=GdhuR2rJJIU

These definitions of a WMD aren't that crazy.