The fact is: just a little extra fat on a dog during a hard competition can cause it to overheat quickly, and/or simply slow it down. Take a greyhound for example; these dogs must be lean and healthy to be able to run like they do. Please check out some pics of conditioned greyhounds if you get a chance. A conditioned dog is NOT an underweight dog. It isn't the same as body-building competitions. A sporting dog is conditioned for performance, not for looks. A good conditioner will find the perfect performance weight for a given dog, and not go an ounce under it. The athletic canine must be strong, fast, and agile, with a high degree of stamina to compete in some of the more grueling competitions, such as weightpull, hog-hunting, agility, racing, etc. An underweight dog will generally be too weak, slow, and unhealthy to be much of a competitor.
And yes, certain poses and lighting can most definitely make a dog appear more "ripped." A dog pulling against a lead while exhaling will absolutely be a different picture than one of the same dog eating or laying around
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Here's another pic of the male in the first picture:
Also, two different pics of the same female, taken the same day: