I was under the wrong impression, thought it meant more traction with "more wheel"but now i see how you explain it!
You are not completely wrong in your general understand, in that the 21" wheels do provide more traction than the 19" -- and, therefore, more resistance for range purposes. The 21" Pirellis are softer, for that very purpose, and optimize performance in Launch mode over the 19" wheels.
I purchased my DE with 19" but also purchased the 21" to complete the exclusive DE "look." I loved the 19" ride, without any reservation. The 21" are noticeably more noisy, and provide a more hard road ride -- but they DO grip the road, without a doubt.
Another consideration against the 21" is a function of the climate where you will be driving. That tire is truly a Summer Run tire only. It is not all season. Aside from all the concerns about pot hole damage to tire and wheel (all true enough), I have had to park my vehicle for a couple of weeks when the ambient temperature dropped below 40 degrees. Pirelli's warranty specifically limits coverage if driven consistently below 40-45, and warnings abound about sidewall cracking under consistent low temperature use. You also experience reduced braking efficiency as the tire material hardens in the cold.
For all these reasons (along with others noted elsewhere in this forum), I plan on switching back to my 19"s and storing the 21"s! Good luck with your decision.
One more thing, if you get the 21" wheels, understand that the front tires are a different size from the rears. On the other hand, the 19" are all the same.