Fit Problem: One cup fits, the other does not

The short answer: Most people have one side up to a half-cup larger; sizing to the smaller side squeezes the larger. Fit the larger side and even up with a removable cookie pad or a slightly shorter strap on the smaller side.

Almost every body has a size difference between sides; studies of fittings put a visible difference in the large majority, commonly around half a cup and often more. So asymmetric fit is not a defect to fix but a default to plan for, and the plan is settled fitting doctrine: fit the larger side, always. A cup that contains your larger side leaves the smaller slightly roomy, which is manageable; the reverse squeezes tissue all day.

Evening up the smaller side is optional and easy when wanted: removable push pads (cookies) in the smaller cup, a half-turn shorter strap on that side to snug the cup closer, or simply stretch and soft-cup styles that flex to each side's volume and make the difference invisible. Molded rigid cups are the unforgiving ones; they hold their shape whether or not tissue fills it.

A new or rapidly increasing asymmetry, as opposed to the stable one you have always had, is the one variant worth mentioning to a clinician at your next visit, not because it is likely to be anything, but because change is the thing worth checking rather than the difference itself.