They're not always kept like that, though, right? I thought I remembered that from the other thread -- that's what they look like when conditioned for a specific event.
So, on the subject of lean dogs, there's a big article on the NYTimes site right now about all of the health benefits of decreasing calories. Animals have been shown to live much longer if their food is limited and they are kept lean. Exercising is also beneficial, of course, but food reduction alone (as opposed to decreasing food less and increasing exercise such that the actual caloric "intake" is the same as the significantly reduced calorie program) is actually the best in terms of increasing longevity and slowing intrinsic aging. The article was about a study being done with humans (reducing caloric intake of average weight and slightly overweight adults by 25%) and was based on studies done on animals. I thought it was very interesting.