Not quite so much as you might think. The engineers are also building to protect their warranty. As we all know, its a lot easier to burn up a saw with a bar that is overworking the engine. What you can do isn't necessarily what will keep warranty claims at a minimum.
The marketing department definitely has a hand in what size bar gets put on the saw. I got a tale on that very topic: My brother-in-law was very proud of his new Craftsman saw with a 24" bar. We were planning a tiny little project in the woods, and he assured me that his new saw would outperform my Shindaiwa 488 with the 18" bar. I said little, but suggested we wait and see. I assured him that bar length doesn't have much to do with the effectiveness of the power-head; that the marketing department is fooling the consumers into thinking you buy chainsaws according to the bar length they are sold with.
At the end of the day, he no longer scorned my little 488, and decided to return the Craftsman and get a 488 Shindaiwa. Which, incidentally, is still running strong 30 years later. Mine, that is. I don't know about his saw at this point.