The World Geodetic System is a set of international standards for describing the shape of the Earth. In the latest, WGS-84 revision, the Earth's geoid is approximated to a reference ellipsoid which takes the form of an oblate spheroid with semi-major and semi-minor axes $a=6378137.0\;\mathrm{m}$ and $c = 6356752.314245\;\mathrm{m}$ respectivelty.

Use the formula for the surface area of an oblate spheroid, $$ S_\mathrm{obl} = 2\pi a^2 \left( 1 + \frac{1-e^2}{e}\mathrm{atanh}(e) \right), \;\mathrm{where}\; e^2 = 1 - \frac{c^2}{a^2}, $$ to calculate the surface area of this reference ellipsoid and compare it with the surface area of the Earth assumed to be a sphere with radius 6371 km.