The volume in question was actually that of the classical "sphere of fixed stars" -- I input it initially as something like "volume of the solar system" and had a brief impulse to accept its rephrasing as "planets volume", especially when I couldn't drill down further into its explanation. It's pretty natural to just assume it knows what you meant.

That query calculates "median planet volume / volume of a small grain of sand", as mentioned by the grandparent poster. I believe it is still an open question how to get Wolfram|Alpha to calculate "total planet volume / volume of a small grain of sand".