Sully did 9 reps at 185 at the combine. He would need to do about 25+ at 225 lbs. to play the line. That's a massive difference. He doesn't appear to like working out enough to add that much strength. .
So true, he has bulk, and it helps not being moved> But we have seen Milsap and Horford push him some this series despite being smaller, they are stronger.