dirt

Hello people. I am new here. I couldn't find a topic like this so please go easy on me if there was indeed a discussion earlier. When I watch nature channels, I wonder how come the herbivores like rhinos and hippos are so muscular. I don't think their size can be all fat. Or is animal fat non jiggly? It is understandable if large cats are muscular as they would be hunting all their lives and will be physically fit but even with them, some younger adults who haven't quite yet started hunting look very muscular! What do you think is the reason? Do these animals unlike us get muscular "naturally"? Anyone having experience watching them up close?