A silly question, but here goes; Why is a concave stomach a sign of being in shape for dogs, but in wolves a bad thing. Is this purely a question of food availibility, or is there some physiological difference there?
I’m honestly not sure entirely what you’re asking about, but I think you might be talking about the part of the dog that is called the tuck.
This is a photo of Fawkes, owned by @impish-iggies. You can see that he’s got a pretty deep chest and then his torso tapers off so that he’s a very narrow dog right at his withers. That little narrow point right before his back legs is called thet tuck. This is a correct look for sighthounds like Fawkes because when he’s running at full extension and needs to get his legs under his body again, having too much bulk there would hinder his movement.
However, this isn’t correct for all dogs.
This is a lab from a UK kennel. (Yes, he doesn’t really look much like pet labs - this is the breed standard conformation). Notice that he has a deep chest and basically no tuck. This is correct for the breed because they don’t gallop like sighthounds - they need to be able to push through vegetation and get lots of power from a sturdy, compact body.
This Grey Wolf does have a bit of a tuck, although it’s obscured by the fur. It’s not as intense as that of a sighthound, but it’s still present. The wolf doesn’t have a lot of chest, so while it has a slight tuck it doesn’t appear to be as distinct a difference as in other dogs. I wonder if you’re thinking of photos where wolves are severely underweight, and so they appear to have much more of a tuck than they should.
Here’s an Ethiopian wolf, for contrast - definitely a very different body structure, with a very short chest and minimal tuck.