Personally, I wouldn't be interested in a midsize chicken. Chocolate Orpingtons may not be an approved color yet, but it very well could in the future. I think if midsize versions are being bred, it is possible that they will get mixed in with either the bantam or the large fowl in the future...
Here are a few pictures of our Chocolate Orpingtons. We need to get some pictures on a bright, sunny day, as the sunshine just brings out the chocolate shade that much more. I can hardly wait until summer.