Right. I'm not sure I follow all the "Smart is huge" comments. He's 6'2 without shoes and weighs 227 pounds.
Smart is huge for a PG (his natural position).
I would compare him to someone like Jason Kidd with his combination of size, length and strength. Kidd was able to defend PGs, SGs and SFs at a high level because of his defensive smarts and physical talent. Smart is very similar in that regard.
Unlike say someone like Rubio who, despite being 6-4, is physically weak and struggles to cope with physical task of defending players bigger than he is. Smart, like Jason Kidd, due to his exceptional lateral quickness + exceptional physical strength is very well equipped to defend opponents bigger than he is and be very effective in doing so.