The change in size of a ferromagnetic at the Curie point

The changes of size of iron and nickel at their Curie points are too great to be explained by purely magnetic forces. On Heisenberg's theory of ferromagnetism, magnetization is accompanied by a change in the strength of binding between atoms, and the resulting changes of size of crystals of iron and nickel at their Curie points are of the same order of magnitude as those observed.