Thanks, the support got it. But we found nothing unusual with your model.

The deformation is just due to the fact that the foot is lifted and tilted at the same time. All vertices move along straight lines during interpolation, so they come closer together, giving the impression of shrinking. This is absolutely normal. It can easily be fixed by using more frames. Bones frames are cheap, so there is no reason to animate a scene with only a few frames, as you would do with vertex animation.