Maintaining child component dimensions with instance swap

I’ve seen lots of complaints about child components persisting their original sizing during instance swaps with the parent. I have the opposite problem: I want a child component to keep its size when I swap it in. For some reason I can’t figure out why “Child B” in this case adopts the size of “Child A” when I swap them in a Parent instance – if I detach the Parent instance from the Parent component and swap the Children they adopt their original size.