He's in a better position to contribute.
He went from playing an open end(Jason Taylor) in college to playing closed end(Kevin Carter) in the Pros. It's a significant change in responsibilites and playstyle. Instead of rushing upfield, Roth now has more responsibility towards the run.
He's bulked up some since last year. He reportedly played in the 270 pound range last year, which is very undersized for the position. Despite that, he did a fairly good job holding up vs. the run, and was included in the short yardage defense alot of the time.
At 285 pounds, Roth is in a much better position to play his role, and I think it is a better fit for his skills then doing what he did in college. In college, he was a power guy playing the role of a speed rusher. When he got sacks, he didnt get them through pure speed or fancy spin moves. He came at them over and over and beat them physical power oriented moves and wore them down over the course of the game.