The short answer is that it doesn't matter. In lobster, it doesn't matter. Canners can be beautiful. Jumbos can be beautiful. Market size can be beautiful. It's not about size at all.
In this case, it's about recovery from moult. It's that simple. A lobster can be beautiful if it has recovered from its moult. If you remove it from the ocean when it's fully recovered, it's going to be beautiful. If it's not, you won't be able to do certain things with it.