I think the majority of those chicks are probably crosses, and not pure bred. The ones with the dark legs almost certainly have some Silkie in them.
Generally bantams are quite small. Especially true bantams like Seramas (true meaning a breed that doesn't have a large fowl counterpart). Do you have a picture of them with something to compare their size to. They look kind of big to me. They may even be bantam x large fowl. I know my OEGB, Nankins, and d' Anvers stayed very small. The Sumatras and the Black Australorps got a bit bigger.