IMHO your muscles are as strong as they need to be to do what you do. If I want my muscles bigger, first, I realize that it's almost certainly just an unreasonable desire for something I don't need. If I want it anyway, I think of something strenuous to do. Generally it's gardening and landscaping. Next weekend I'm digging a new pond, and I'll be placing a lot of large stones this summer. I figure it's silly to go somewhere and lift heavy things for no reason; it's about like riding a stationary bike when you could be out in the air having fun.