In a major subdivision application to which both public water and sewer service are to be provided, variable lot sizes may be approved by the Planning Board. The lot sizes may be varied to the extent that the lots may have areas of not less than fifteen thousand (15,000) square feet each and a width of not less than eighty-five (85) feet if a sufficient number of the remainder of the lots are increased in area so that the average lot size will be not less than twenty thousand (20,000) square feet. The Planning Board, in passing on such plat, shall consider the physical and structural characteristics, including the topography of the land comprised in the subdivision, and determine that variation in lot size will provide a better layout of the subdivision and a better use of land for building sites than would a uniform lot size. The subdivider shall submit a plat map showing the development according to the requirements of the R-A district as shown in the schedule, Section 37-63, and another plat map showing the development as modified in accordance with this section. In no event shall the density of development throughout the subdivision exceed the exact number of lots that would have been permitted to be built if such development has proceeded on the basis of twenty thousand (20,000) square feet per lot with a width of not less than one hundred (100) feet.