Rainbow Shark isn’t growing

Hi all! I’m not sure of all the details you need, but I have a 55 gallon tank, it’s cycled and healthy. We’ve had it up for about a year now and we haven’t lost any fish. My concern is we have 2 rainbow sharks that we bought together. One of them have seriously out grown the other. And I mean seriously out grown, it’s the same size it was when we brought it home a year ago, but the other is about 5 inches long now. I’m going to post a few pics. I guess I’m just wondering if this is normal. The one by the rocks is the smaller one. The one by the Crayfish and Castle is the larger one.
Thanks!!

Rainbow sharks are territorial and if you have 2 males, the dominant one will bully the smaller one and prevent it from growing. Male rainbow sharks have a black edge to the anal (bottom) fin.