The estate of the trustee is determined as to its extent by the extent of the interest of the cestui que trust. If the legal estate of the trustee is less in quantity than the equitable estate of the cestui que trust, it will be enlarged sufficiently to enable the trustee to perform the purposes of the trust. If the estate of the trustee is greater than is necessary for the purposes of the trust there will be a resulting trust back to the trustee as to the residue.