I'm not even sure that's the correct term for it.
What I mean is, say there is a fat character in the play (I mean obviously - with 'fat' in their name) - do you cast the most overweight child as that part, or would you think it very inappropriate?
I feel a bit uncomfortable about it tbh.
I wonder how far it should go, really - you always get blonde angels in the nativity but casting a child on the basis of their size seems wrong.