For a constant field in the gap, the motor armature and running characteristics are independent of the size of the gap.
However, to get that constant field, a wound motor needs excitation current more or less proportional to the gap, so the gap affects efficiency. A permanent magnet motor needs magnet strength proportional to the gap, so the gap affects cost.
If the gap is already running at the maximum flux it can, so the pole piece material is at saturation, then reducing the gap further will not increase the field or armature efficiency. It will however reduce the MMF required to maintain that field.
Achieving a very small gap needs high mechanical precision, which is expensive. It's therefore a reasonable tradeoff for the manufacturer to aim for a larger gap, and spend more on magnetic field and less on mechanical precision, when trying to cost-optimise a motor. A larger gap also means slightly less 'windage' loss, from shearing the air in the gaps.