The larger the gap, the higher the potential needed to bridge the gap and produce a spark, and the shorter and stronger the spark will be. The ignition is designed to give you a spark after a specific potential (voltage) is reached, which is determined by the spark plug gap. If the gap is too large, you will not get normal sparking because the ignition will not reliably produce enough potential to jump the larger gap. Too small of a gap will give a longer cooler spark because the energy stored in your ignition (a fixed quantity) will be dissipated over a longer time. As soon as the energy released by your coil as its field collapses is enough to bridge the gap, the spark begins.
This means that in order to work with your ignition, the total gaps in both plugs must be the same. The difference in external gap in the electrodes can be explained by the presence of a larger internal gap in the insulator of the Denso plugs. The total gap (internal + external) in the two plugs should be the same for the same performance, but the external gaps are differerent because the internal gaps are different.
If you get a chance to take an x-ray of the Denso plugs, you should be able to see the internal gap. Also, because of the gap, I bet an ohmmeter will show there is no continuity between the center electrode and the spark plug wire connector at the top of the plug in the Denso.