Lens choice and camera distance.
This is where drawing rules can trick you into forgetting optics.
A close-up portrait always carries perspective distortion. Forms closer to the camera expand. Forms farther away compress. Lines and planes start converging toward vanishing points.
Ignore that, and the proportions will look wrong no matter how carefully you build the head.
Even inexperienced viewers can feel it, because their eyes are already trained by photography, film, games, and every close-up face they have ever seen.