e7k was close to the right answer.
In life, we see in three dimensions. So, when someone's leg is lifted toward us as part of the running motion, our brain sees that leg in perspective. A photograph is inherently two-dimensional (and it freezes a single moment in time) and therefore we don't get the same effect of perspective. So, if I'm looking more or less straight-on at a runner in a photo, and part of the runner's body that is not perpendicular to a line between the camera and the body part appears shorter than it really is, but just as wide as it really is. Thus, the effect of looking heavier in photographs.
If you can find 3D photographs, this problem mostly clears up, but there isn't much 3D photography done these days.
Sometimes, also, you capture a runner with a completely flexed quadricep. And that muscle doesn't normally appear in that position, so it will look thicker in the photograph than when you see the same person standing in shorts in person.
Hope that helps.