It's a conclusion brought about by inference, based on experience with other specimens of the same/similar body shape. If you've read/listened to the DW book The Resurrection Casket, there's a ball-on-sticks-shaped character that gets taken out by a judiciously-flung spheroid. And I think Buffy gets knocked over more when she gets all skinny but still has that massive heed. (Like an orange on a toothpick.)

Steak knives against the Asgard, huh? Hmm. Wouldn't they be kinda tough and stringy?

(hehheheh Wouldn't that be "assault and ZPM? hehehehe *is also dork*)