Smaller ? How can you possibly know how tall the guy is this drawing is, considering there's no one to compare him to ? From what I see, he might be 2, 3 or 7 feet tall, we don't know...
As for thinner, well, I guess you're comparing him to the movie's version of Thorin played by Richard Armitage... But don't forget movie-Thorin is just an adaptation, the "real" Thorin was created by Tolkien and we have no idea how thin/fat he was ! This portrait is the artist's interpretation of a fictional character, there is no "right" way to draw him...
As for thinner, well, I guess you're comparing him to the movie's version of Thorin played by Richard Armitage... But don't forget movie-Thorin is just an adaptation, the "real" Thorin was created by Tolkien and we have no idea how thin/fat he was ! This portrait is the artist's interpretation of a fictional character, there is no "right" way to draw him...