However, if this were the only consideration, you could get a wider-diameter manhole with a smaller cover by making it in the form of an equilateral triangle, as evidently Nashua, NH, does: https://en.wikipedia.org/wiki/Manhole_cover#Other. Indeed, you don't need to stop there; if you curve the sides of the triangle in toward the center, you can get a manhole with an even smaller surface area for a given diameter, although at some point the extra "diameter" will be too curved to be useful.