My Australorp actually stole some out of a box next to her. She had to lift them up several inches to get them over the edge of the box. I'm amazed at how chickens can move eggs. (I almost agree with the arm theory!)
I didn't know they could hold chicks either. You pick them up, and they have almost half a dozen chicks tucked under their wings that you don't know are even there unless you spread the wings and give them a little shake, LOL.
Chicken mamas are so cool.
Oh ... my Austalorp, a BIG girl, couldn't handle more than 3 or so silkie sized eggs, just because she wasn't good at seeing what was going on when she put stolen eggs under herself or changed her position. My silver penciled banty could easily handle 6 or 7 silkie-sized eggs and did a GREAT job of keeping them all under her most of the time. My rosecomb, otoh, who is the smallest chicken of all of them, showed up with 15 chicks last week that she apparently was able to incubate just fine (from rosecomb sized eggs, lol).
Not that that answers your question (and I think others have already done a good job of that) but just to say that in my VERY limited experience (this is my first time hatching any babies!) it seems to depend somewhat on the individual hen as well? At least as far as I can see.
trish