I don't think it's a question of causation. I'm not sure anybody can really be certain that smaller guys with 'unusual' pitching motions - insert whatever word you want for 'unsual' (violent, unorthodox, explosive, etc.) - are more prone to injury because of their smaller stature and the nature of their deliveries.
To me, it's more a question of liklihood of the smaller pitcher having sustained success vs. a physically bigger pitcher.
There are lots of examples of big pitchers who have blown out elbows or torn up their shoulders. If we knew why it happened, it wouldn't happen nearly as much as it does. The list of small pitchers who never got drafted, never made it out of A, AA, AAA ball, because they were too small and didn't have the velocity or the stamina or any of the other characteristics that come with being physically large, is quite long.
I would venture a guess that the ratio for success vs. injury and nonperformance is greater for bigger pitchers than it is for smaller pitchers. Of course it's possible to find that rare guy that is able to do what Maddux and Lincecum and Oswalt have managed to do. It's just not likely.
But having said all that, I'd take a chance on him because we've got 3 first round picks and if the team's scouts are sold in him being one of the exceptions, I"m fine with it.