Generally, manufacturers specify a large gap to resist fouling. Depending on the tuned condition of an engine and running conditions, you can get by with a smaller gap.
A small gap requires a lower voltage to arc the points so there's less stress on the coil. The distance the spark has to travel is one factor in determining how much voltage is needed to arc across the plug... if everything is right, it can work fine.
But a small gap fouls easier, so if there's water or too much oil in the mix or if the mix is too rich, it can foul. A tiny drop of something can bridge the gap. Another thing is length of the spark is shorter, which is ok as long as the engine is in tune and mixture ratio is correct, especially at low rpms.
A high performance engine with high compresion ratio at high rpms would call for a smaller gap for best performance, but the ignition has to be fast and healthy enough to provide it under those conditions.
Keep the plug clean and adusted and everything else in tune .. and a small gap won't be noticed.