I weigh as much as a duck

Is "I weigh as much as a duck" an idiom? If yes, what does it mean?

I have never heard it, although it may be a reference to the film "Monty Python and the Quest for the Holy Grail" in which there is a funny pseudo-science scene in which a noble scholar claims witches can be identified because they are "made of wood" and float "like a duck."