I'm pretty sure the images are scaled differently (note the sizes of the bones, which should be the same). The sedentary person isn't just sedentary, they look

I'm pretty sure the images are scaled differently (note the sizes of the bones, which should be the same). The sedentary person isn't just sedentary, they look to be morbidly obese. That image is zoomed out, but the actual size of the muscle isn't that different. It's just surrounded by a huge amount of fat.