[Smokey Joe]

S. D. of a bullet is its weight in pounds divided by the square of its diameter in inches. Well, so what??

This is what: "The higher the S. D. of a bullet in a given diameter, the heavier and longer that bullet will be. This is important to several factors, including the B. C. of the bullet. However, from a big game hunter's standpoint the most relevant measure of S. D. is that with all else being equal, the higher the S. D. of a bullet, the better its ability to penetrate."
Lyman's 48th edition Reloading Manual, pg. 105

Same source: "The ability of a bullet to retain its velocity is expressed as the ballistic coefficient. This is a measure of how aerodynamic a bullet is and how well it cuts through the air." The higher the number, the better the bullet slips through the air, and the less velocity it loses over a given distance.

Bottom line: All else being equal, longer, heavier, pointier-nosed bullets go through both air and targets better than shorter, fatter, rounder-nosed bullets. Simple, no?