Rainbow Dash enters the big head cheat in style. I did not edit this; I think QA was falling asleep during this scene. For comparison to the correct head size proportion, see >>56586. This happens to Cheerilee, too.

Basically, for this mountain scene, the animators were using a relatively smooth walk cycle for the lower body while attaching the moving heads. Then they made a bit of a derp attaching the heads at this point.