To be eligible for original incorporation as a municipal corporation, the area embraced shall be so developed that at least 60 percent of the total number of lots and tracts in the area at the time of incorporation are used for residential, commercial, industrial, institutional, or governmental purposes and shall be subdivided into lots and tracts such that at least 60 percent of the total acreage, not counting the acreage used at the time of incorporation for commercial, industrial, governmental, or institutional purposes, consists of lots and tracts of five acres or less in size.