Yeah, from the descriptions I'm guessing that Andi is a wee bit smaller in the chest than Sally, who is a bit smaller than Cynthia.  So swapping with either other girl will leave the bigger one with a gappage problem. :)  Or at least, it'd be more fun if we made it that way.  For simplicity we can think of Andi as a A, Sally as a B, and Cynthia as a C -- and if we get Jim some hormone-magic maybe he can complete the list as a D, big guy!