They are probably Easter Eggers, but yes, they come in both large fowl and bantam size. There are a lot of threads on Ameraucana vs EE, but if you got them from a feed store or hatchery, then they are Easter Eggers, not Ameraucanas. Hatcheries often call what they have Ameraucanas, but they don't meet breed standards. I have both EE's and show quality Ameraucanas, and I love them both. :)
As far as your chick goes, though, I have had chicks that didn't grow before. Most have died, but I've had some make it and grow up to be normal size. If you got them together and they're supposed to be the same thing, then I would guess this is the case. I always just called it failure to thrive, like some kids of friends have had as babies. They all grew up normal and healthy, but it's harder to deal with in an animal. Personally, for me, I don't have the time to worry and take care of a FTT chick, so they either make it or they don't. If you are concerned and have the time, then try to hand feed the chick extra high calorie treats. Like maybe some canned cat food. But if it dies, it was nothing you did wrong, it just happens sometimes. Good luck and enjoy them!