If you take objects of the same material-- that means they have the same density, the density of the objects that you drop in the liquid or that you drop in the gas-- so that m equals 4/3 pi rho r cubed-- this is now the rho, the density of the object; it's not the density of the medium-- then you can immediately see, since you get an r cubed here, that this is proportional to the square of the radius if you drop objects in there with the same density. 