Why is home base such an odd shape?

http://mlb.mlb.com/mlb/downloads/y2008/official_rules/01_objectives_of_the_game.pdf 1.05 Home base shall be marked by a five-sided slab of whitened rubber. It shall be a 17-inch square with two of the corners removed so that one edge is 17 inches long, two adjacent sides are 8½ inches and the remaining two sides are 12 inches and set at an angle to make a point. It shall be set in the ground with the point at the intersection of the lines extending from home base to first base and to third base; with the 17-inch edge facing the pitcher’s plate, and the two 12-inch edges coinciding with the first and third base lines. The top edges of home base shall be beveled and the base shall be fixed in the ground level with the ground surface. 1. Notice that it is defined as home base, not home plate. 2. Why not simply make it a square? The other bases are 15 inch squares. Home base could be a 17 inch inch square positioned like second base. Or 15 inch. Why is it a different size than the other bases as well as a different shape?