The wider the gap, the larger the spark kernel, and the higher the probability that there is a combustible mixture at the location of the spark and that combustion is started. Also, the wider the gap, the more spark voltage is required to jump the gap, so there's a limit - the voltage generating ability of the coil(s).

While playing with the turbo Raider, I kept getting high rpm random misfires. I found a site that suggested that the VERY high cmbustion chamber turbulence in a blown motor was blowing out the spark before it fired the mixture. I reduced the gap from .039 to .032, and the misfire went away. Note Frank's .035 reco above...

The gap has to be wide enough so that the coil voltage builds up high enough to both jump the gap and be hot enough for a very high probability of combustion. Thus a narrow gap is too cool a spark..