more boost = more combustion O2 avilable. Add a corresponding amount of additional fuel and you have a more powerful combustion event. Even though the dynamic compression ratio remains the same +/- , the lower height of the 18-1 piston crown creates a larger effective combustion chamber volume, which is now packed with the same density of air/fuel as the smaller area that was provided by the stock piston compression height.

Same DCR + Same A/F Density+ more A/F Volume = bigger bang per power stroke