I have never played quarterback, so I don't know this from personal experience, BUT...
It looks to me that Dekkers has the physical gifts (bigger / stronger) to throw the ball with more velocity and distance with less effort than Purdy.
Purdy was GREAT. I really appreciated everything he did and don't regret any of his time here, But....
IF and when he threw further downfield, it took a lot of effort, needed to be well blocked and was not consistently done on the run or outside the pocket later in the play. It took ideal circumstances for a longer play to be pulled off. I attribute this to not having all the strength required to pull it off in less than ideal circumstances. If the long throw is a flick of the wrist vs. having to engage your entire body to make it, then you can concentrate on the accuracy vs. mustering all your strength to get it there.
That is why I believe Dekkers has a better shot at stretching the field. His long throws look effortless. Purdy's did not. Once again, I'm not taking anything away from Purdy, he was a great quarterback, that is just a difference I see.