Bigger is by no means better in everything. Sure, stretching an aircraft allows for increased passenger capacity, but most of the time it also decreases the airplane's range by a noticeable degree. For example, the 757-200 is capable of flying transatlantic routes and transcontinental U.S. runs nonstop, but the -300, while accomodating many more passengers, has considerably less range than the -200. Likewise, the A330-300 has considerably less range than the -200, and so forth. NW
perfectly demonstrates their arrangement of the 757 types and A330 types. The -200s are used on the transcontinental to medium routes, while the -300s are flown on shorter high density routes such as DTW
. The A330-300 is used on the European routes exclusively because loads are higher on their European routes and because it lacks the long range needed to cross the Pacific. The A330-200 resolves the range issue because it is lighter and therefore able to fly farther....passenger capacity may not be a factor here because the A333 cannot fly SFO
nonstop. I hope this helps.