The mayor and council of such city, with the consent of a majority of the legal voters of such city voting at an election thereof, shall have power to extend the limits of the city over territory adjacent thereto, and to diminish the limits of the city by excluding territory therefrom, and shall, in every case, have power, with the consent of the legal voters as aforesaid, to extend or diminish the city limits in such manner as in their judgment and discretion may redound to the benefit of the city; provided, however, that no election or voter consent shall be required for voluntary annexations or transfers of jurisdiction under chapter 71.