Interestingly enough Bob Voulgaris was just on the Bill Simmons podcast and asked a similar question. He was pontificating on if Smart was the best defender on this team. He basically said Smart is a really good defensive player, but he finds it hard to believe that he makes as much of an impact as Tatum and Brown defensively due to size.
Something that made so much sense to me: Simmons said that Smart has such a knack for making big plays at such important times like: Diving and getting a ball, taking a charge, saving a ball, etc. Voulgaris said yea I call those 'spotlight plays'. I'd never heard that term before, but thought it was brilliant.