jmc

We know that the two spheres are similar (in the same sense that triangles are similar) because corresponding parts are in proportion. We will prove that for two spheres that are similar in the ratio $1:k$, their volumes have the ratio $1:k^3$. Let the radius of the first sphere be $r$, so the radius of the other sphere is $kr$. The volume of the first sphere is $\frac{4}{3}\pi r^3$ and the volume of the second sphere is $\frac{4}{3}\pi (kr{)}^3$. The ratio between the two volumes is $$\frac{\frac{4}{3}\pi r^3}{\frac{4}{3}\pi (kr{)}^3}=\frac{r^3}{k^3{r}^3}=\frac{1}{k^3}$$ Thus, the ratio of the volumes of the two spheres is $1:k^3$.

In this problem, since the smaller sphere has $12.5\%=\frac{1}{8}$ of the volume of the larger sphere, the radius is $\sqrt[3]{\frac{1}{8}}=\frac{1}{2}$ that of the larger sphere. Thus, the ratio between the two radii is $\boxed{\frac{1}{2}}$.

(In general, the ratio of the volumes of two similar 3-D shapes is the cube of the ratio of the lengths of corresponding sides.)