How does the gap between coils and magnets affect BLDCs
In any given motor, a higher field means better efficiency, with a lower current needed for any given torque, so lower copper losses. It also means better power per volume/cost/weight of motor.
The permanent magnets have to set up that field across the airgap. With a longer airgap, you need stronger (aka more expensive) or longer (aka heavier, more expensive and bulkier) magnets than with a motor that maintains a shorter airgap.
To maintain a small airgap without clashes between poles requires a more accurately made (more expensive) and stiffer motor.
There is a small penalty in very tight airgaps that the mechanical loss due to air viscosity increases, but that's generally outweighed by the reduction in electrical losses.
The design of any given motor is therefore a compromise between several different expensive parameters