Dogs haven't evolved unimpeded, they've been selectively bred and with a generation length significantly shorter than humans'. They've only changed in so much as humans have attempted to draw out specific traits and suppress others. There are still plenty of mutts, plenty of wild dogs, and the pure breeds who are artificially selected tend to have as much wrong with them as right with them. Not a great model for human evolution, I would say.
I don't understand the idea of "inefficient muscle." Yes, there are different types of muscle fibers and they preferentially burn fat or carbohydrates, but that has more to do with what type of movement is being done rather than a person's weight. If someone has trained their body to perform aerobically, they will be building up muscle fibers that burn fat over glycogen. I would say that's probably the goal of most runners, but it's not the exclusive goal. We still need those sprinting muscle fibers, which burn primarily glycogen, we just want them to kick in at higher speeds. All of that can only be achieved by training.
But an obese person who does not move much shouldn't have "inefficient" muscles, they just have muscles that are efficient to their purpose (and not to, say, running). Most of the calories we burn go towards our basal metabolic rate, our basic daily functions that won't stop until we're actually starving. An obese person's BMR is generally considerably higher than a thin person's, because a) they have a lot more body to function in those basic ways and b) they even often have more lean body mass (not by percentage, but by raw weight) than a thin person just by the necessity of moving around their own heavier body. Why would an obese person's muscles (and other tissues), performing basic functions, be less efficient than a thin person's? Those muscles are "in training" just as much as anybody else's. So the idea that if you take two people, of greatly varying sizes, who perform similar movements throughout the day, you would find the thinner one has a higher metabolism or can eat more and maintain their weight, is simply ludicrous.
However, many people claim it. I would imagine it's simply because they have no idea what they're eating. An old roommate of mine, a 5'8" sedentary woman who weighs around 250 pounds, once said to me, "My body knows exactly how much to eat, I stop eating when I don't need anymore calories, and I don't have any trouble maintaining my weight (I just can't lose it!)." She took this as evidence that she was eating well enough. Well, thank goodness she could maintain her weight (i.e. not gain), but she could only maintain her weight because she ate an extremely high number of calories each day, probably twice the volume of what I ate, and had absolutely no realization of that. Heavier people eat more simply to maintain their weight. All of that food goes primarily to basic metabolic processes which do not differ significantly from person to person. The fact that if they walked a mile they'd burn a higher percentage of sugar than fat is irrelevant to that issue. They'll still burn more calories walking a mile than someone who weighed less.
