My conclusion is that as long as the captain is reasonably competent, and the stoker has good balance, there is no reason why a smaller captain cannot ride with a larger stoker, easily.
I'm not sure I'd completely agree with the "easily" part. Larger stronger Caaptain has a big advantage. Any problems with coordination/ balance, a larger stronger captain can overcome with brute strength.
A smaller captain that's a good rider, and a stoker who's working well with the captain as a well cooridnated team will be fine, but most people don't start out that way.
I think most people will find it easier, particularly starting out riding a tandem, for the larger stronger person to be captain.