I am thinking it is because you have a bigger spark with a larger plug gap that you want .025 inch instead of .019 or.022. Right? But what is the problem if the engine runs better (read OK) at a plug gap of .019? Any downsides? Thanks.
That is true till you add in pressure. when you increase the pressure of the air fuel mix that the spark has to jump it increases the resistance and in effect you have a smaller spark. to offset the increased resistance one would decrease the gap that the spark has to jump across. so one could say that the size of spark is inversly proportional to the pressure at which it has to jump a set gap.