Mine were all healthy, I got the 25 rainbow egg layers pullets. 26 birds were sent, one of which (so far) has turned out to be a roo, and I got no EEs. Most of them are either brown or white leghorns, with some SLW and BA in the mix. A few dom's as well.
The one chick that you have that is smaller is probably just having a 'failure to thrive' issue. It could go on to be a happy healthy bird, but it could also die. One way to tell if it is bantam or standard is to remember how they were when you got them. If it is a bantam, you would have noticed then. Day old bantams are maybe half as big as day old LF, you should have noticed a big difference!
I would say it's just failure to thrive. Just be sure that one is eating enough, that the others aren't pushing it out of the way at feeding time.