Orpington are supposed to have white legs but often with black birds the pigment can cause the legs to be black or at least dark. You can see that if you look through these Orpington photos on Feathersite. This can happen whether the legs are yellow or white. To me the black legs do not definitively tell me that it is not an Orpington.
http://www.feathersite.com/Poultry/CGK/Orps/BRKOrps.html
Also it is a hatchery chicken. You seldom get a chicken that meets the breed SOP from a hatchery. Part of that is that they are not in the business to produce show quality chickens. They are in the business to mass produce chickens that sort of follow the breed characteristics, close enough for backyard purposes and at an affordable price. Each hatchery has different people determining which chickens get to breed, some with doctorates in poultry science and real knowledge of the SOP. But each hatcheries' person will select for traits they think are important. That's why you can get differences by hatchery.
A big difference is the breeding method. A breeder usually selects one specific male and one or two specific hens to give them the best chance of producing a chicken that follows the SOP. Even then the majority of chicks don't measure up. Hatcheries tend to use the pen breeding method. They might have 20 roosters in a pen with 200 hens with random matings. Even if the person selecting the breeders follows the SOP really closely you are not going to get many chicks that meet the SOP with this random breeding. If you want chickens that closely follow the SOP get them from a breeder, not a hatchery, and pay the price the breeder demands.
With all that said, the black does not look that Orpington-ish to me, but best I can tell the buff and the male leave some to be desired too. I've gotten chicks from Meyer Hatchery, not Orpington but other breeds. Most of the individuals I got were closer to their breed SOP body type than that black is to Orpington, but I also got a lot of variation between individuals. That's been true from other hatcheries too, I see a lot of differences in individuals.
Is that black within the limits you might get from a hatchery Orpington? It's stretching it but maybe. It's possible it's a mix-up at the hatchery, the Black Orp sorting pen should be at a different location than the pen for Buff or Blue Orps. I'm sure things get really hectic on that sorting floor when they are trying to ship out tens of thousands of chicks that day. It's even possible a Black Jersey Giant rooster got loose and wound up in a Black Orpington pen instead of a Buff Orpington a Black Rock pen, but that is a real stretch, highly unlikely.
Looking at that photo, many things look female. But with that curving top tail feather especially it is also possible you have a late developing cockerel as someone else mentioned. That would go a long way toward explaining the size. To me this is the most logical explanation but it is only a guess. The Orp feathering could develop more as he matures.
