I think he needs to work on his handle in the offseason. He may never be super quick, but I think with improved confidence and deceptiveness with his dribble drives he will be able to drive to basket more successfully than he does now.
Yeah, and Paul Pierce might be the best possible example of this.
Pierce was never an incredibly quick or athletic guy, but he was able to use his size/strength, handle, footwork and ball fakes to get to his spots as well as pretty much anybody in the league.
Even though Pierce didn't have the outright agility or fancy dribbling moves, Pierce was very effective as a point forward late in his career because he had a safe handle, good BBIQ, good court vision, and the size to see over the defense.
I think once Smart learns how to use his strength, footwork and ball fakes to create space then I think he'll be able to be very effective on offense. I can see him becoming a PG equivalent of Paul Pierce, albeit being more of a defensive guy who can hold his own on offense (while Pierce was an offensive guy who could hold his own on defense).
Also as others have said, Smart may not be the most ripped guy out there (I'd guess he's probably around 10% body fat) but he's definitely not fat or chubby the way big Baby and Sully are. He's got a LOT of muscle on that frame.