When I say "size" I don't mean what a scale says, I mean how big he plays. Fairley is a LOT like Glenn Dorsey and we know how much Dorsey has struggled in the NFL. Both are 300 but neither one plays like it. Finesse DT's just don't get it done at the next level and there is tons and tons of evidence for that.
On the flip side you have guys like Kendrick Clancy or Jay Ratliff, who are about that same size at 300 pounds but play like guys that weigh 350.
Regardless, there is a reason that nobody in the NFL has much talent at defensive tackle, and it's in large part because more than half the players drafted are huge busts.