Knockback - why ST not HP?

[Tomsdad]

To be honest GURPS isn't consistent with how it deals with mass, so I;d substitute what ever stat gives you the result you want.

If you want ti have high ST low Mass supers knocking each other about like ping pong balls, than work out what a "normal" ST/HP stats would be for their Mass either using the weight/height tables, or HP=2x(cube root of mass) for living creatures (IIRC)