Hi, I'm new here. I ordered three girls...a Buff orp, a SL wyandotte, and an Austrolorp. These were part of a larger order at the feed mill through Murray McMurray. My friend has four chicks she ordered at the same time and mine are developing much faster than hers...she has a Favrolle, a Brahma, an Americana and a Silkie (which of course is smaller.) Are my breeds known for getting big really fast, or do you think there may be some other issues such as gender or growing conditions. Mine are box-brooder raised and hers are being raised by a broody silkie hen (too cute by the way!!!) My Buff orp in particular is a porker...a nice one, though. I'm thinking they look more like 5-6 week olds instead of the 3 1/2 week-olds they are. And we cannot have any roosters.
I think your breeds are just bigger birds. I have 2 of each of those breeds and they are bigger than my others also which are ameraucanas.
Also, hen-raised chicks tend to be slimmer overall. They expend alot more energy keeping up with mom and they eat more bugs and greens. Brooder-raised chicks just stay in a box and pork out on commercial feed and get little exercise. I just has this experience with a broody raising two Barred Rock babies-they seemed much smaller at six weeks than all my previous brooder Barred Rock babies.