Nonconforming lot sizes. Whenever there shall have been legally created, prior to September 1, 1971, or as a result of transfer prior to said date, a lot of lesser width than required by the terms of this chapter for the district in which it is located, the minimum side yard width of such lot shall be proportionate to the side yard width otherwise required within such district, as the width of such lot is proportionate to the minimum lot width otherwise permitted in such district.