Kg is the measurement of mass. Its just purely coincidental that under normal gravity conditions of g = 9.81 m/s^2 mass and weight are equal. So even tough your scale strictly speaking measures weight, what it spits out is still mass.
So a object measured on earth weighting 2 Kg has a mass of 2 Kg. Now the rest is really up to what kind of units you decide your 3d world has.
Ps: a virtual ball allways weights very little close to 0 in fact, it can have a virtual weight, but thats for you to decide