No he's not tiny. My gracie turned a year in July and she's 20lbs. They grow and fill out and there own pace. Thats actually a real good weight for him.
Maybe a tad on the small side but nothing I would worry about. He's still got a lot of filling out to do. My cardi seemed to be a really slow grower around that age but then shot up 6 lbs. after he turned a year old.
You could move him to an adult food at his age too.
Johann is almost as old as Koda is and weighs about 19 lbs too, but I knew he was going to be smaller than average because his parents were also like 20 lbs each. Do you know much about Koda's parents? Or like others have said, he may just grow at his own pace!