Why are some models of airplanes more efficient for carrying cargo than others? For example, the A332F is better than the A333F (if there ever was one) as far as weight-carrying capability. The 763ERF is better than the 764F. But in both of those examples, the larger airplane has more room inside than the other, so why wouldn't manufacturers build the larger model of that series to suit the needs of a cargo carrier. The 744F is capable, the MD-11F is capable, so why can't the 764 be capable or the A333 be capable?
One last example is the new Boeing KC-X tanker, which IIRC is built on a 762 frame with winglets. But wouldn't a 764 with the raked winglets and longer fuselage hold more cargo and fuel and still be able to go just as far if not further?