It's Usually Position, Not Size
Most people assume a higher cheekbone is a bigger one. It rarely is. In most cases the two cheekbones are close to the same size, but one sits in a different position. And "higher" is often really "further forward" or "tilted," which the eye reads as height.
That is the first reason it's hard to pin down from a photo. Several different structural patterns land in the same place: one cheek sitting high.
The Patterns That Push a Cheekbone Up
**Lateral strain.** The sphenoid, the central bone the rest of the face hangs off, can shear toward one side and push that cheekbone forward. A forward cheekbone catches light differently and reads as higher, even when its actual vertical position is close to the other side.
**A maxillary cant.** The upper jaw can tilt so one side rides higher, carrying the cheekbone up with it. This usually travels with the bite, since the upper teeth and the maxilla move as one piece, so an uneven bite height and a high cheekbone often show up together.
**A tilted atlas.** The head balances on the top vertebra, the atlas. If it tilts, the face tilts with it. Sometimes the cranial bones shift to keep the eyes level and the cheek follows; sometimes the atlas tilt is itself compensating for something lower in the spine. Either way one cheekbone ends up higher.
**Temporal bone rotation.** The arch of the cheekbone is finished by a process of the temporal bone. When that bone rotates in or out it lifts or flattens the cheek on that side, and it rarely moves without dragging the sphenoid and occiput along with it.
**More than one at once.** Often it is. A side-bending rotation lengthens the open side of the face and flares its cheekbone outward through external rotation. Layer a lateral strain on top and the squeezed side gets pushed forward, so that side can end up looking higher, the reverse of what the side-bending alone would suggest.
When the Bone Is Genuinely Smaller
Occasionally it really is a size difference. In hypoplasia, one zygoma or one side of the maxilla simply developed smaller. This is the exception to everything above. The bone isn't out of position, it's short of volume, and unlike a strain pattern it doesn't shift back into place.
It's worth knowing which situation you're in before assuming a cheekbone can be moved, because a bone that never moved out of place is a different problem from one that grew small.
Why the Mirror Can't Tell You Which One
From the front these look nearly identical: one cheek sitting high. But a forward sphenoid, a canted maxilla, a tilted atlas, and a rotated temporal bone are different problems with different origins, and they routinely stack on top of each other.
Which one you're actually looking at depends on how the whole skull is sitting and how it has been loaded over the years, not on the cheekbone by itself. That is the part a mirror, and most of the advice online, quietly skips.