In equal parallelepipedal solids the bases are reciprocally proportional to the heights; and those parallelepipedal solids in which the bases are reciprocally proportional to the heights are equal.
Proof
3D analogue of VI.14: by XI.32 the ratio of volumes is the
compounded ratio of bases and heights; equal volumes force the
compounded ratio to be unity, i.e.\ reciprocal proportion.