Spark Plug Basics
The straight scoop on small diameter/thinner electrode plugs is this -- 2 main benefits:
1) Less voltage needed to initiate gap ionization.
2) More resistant to wet fouling.
Now, on to why the thin electrode plugs cost so much:
Electrons have a property known as "electron crowding". It can be compared to the distribution of people as an elevator becomes more and dense. Nobody wants to rub elbows with a stranger so they re-shuffle to get maximum personal space until they all finally feel like they just gotta get the heck out of the box. In the same manner, (but based on electrostatic repulsion) the smaller tipped an electrode becomes, the less voltage is required to make electrons want to make the jump to the ground electrode. This also contributes partly to the fouling resistance.
The second part of the wet fouling resistance goes like this. As a center electrode becomes carbon coated and then fuel wetted, it can "bleed off" part of the electron charge that is supposed to provide pressure at the electrode tip by providing a path down the electrode and over the porcelain insulator to the grounded body of the plug. Smaller electrodes have less surface area, and that alone limits how much wet conduction of the charge to ground can occur.
Ignition-like CDs (magnetos too) have much faster "rise times" than older, mechanically triggered or inductive ignitions do. These were developed to combat the wet fouling also, by giving a faster rise to peak voltage (shorter rise time). They are much more likely to provide enough voltage to initiate a spark before the voltage can be bled off to ground via the wet electrode and insulator. The thinner electrodes are also advantageous because they shorten the rise time by lowering the necessary voltage needed for gap ionization while having less surface area to bleed voltage off in the first place.
The plug's electrodes are subjected to the full heat of the combustion process, and they need to get rid of that heat load to survive. A thicker electrode has a larger heat sinking path to move that heat away from the tip...so thick electrodes can be made of cheaper metals. The thinner the electrode becomes, the more heat-resistant the metal of the electrode needs to be. Heat also affects the electrical resistance of the metals that are conducting the electrical charges.
So, the metals that are resistant to both heat AND remain effective conductors at elevated temps cost a lot more than the more common metals that can be used in fat electrodes.
Here's a little trick that can help someone out if they have a wet fouled plug while out riding: pull the plug cap off and re-set it so that there is about a 1/4" to 3/8" gap between the cap's metal connector and the tip of the spark plug, and then start the machine. When it is fired up, simply push the cap back on to normal fitment.
What this does is ensure that the spark energy has reached its full potential voltage before it gets to the plugs electrode, effectively making for zero rise time to peak voltage as far as the plug gap is concerned with no chance to bleed off voltage before it hits peak. The wet plug will usually fire, and once it's running the plug cap can be re-set to normal.