Size doesn't matter in terms of quality. Of course, the bigger the fish the more meat you get. But a twelve pound salmon is just as good as a twenty pound salmon, all else being equal. And that's where "quality" matters. What determines quality? How it was caught, how it was kept cold or alive for how long, how it was transported and how quickly, how it was cooked. So a small fish, treated with great respect, is a better quality fish than a very large fish treated poorly.