Moment of inertia of a solid sphere is $I = (2/5)MR^2$. Mass $M = \text{density} \times \text{volume} = \rho \times (4/3)\pi R^3$. So, $I \propto R^3 \times R^2 = R^5$. The ratio of moments of inertia will be $(R_1/R_2)^5 = (1/2)^5 = 1:32$.