If Dean was the small one, he’d still be in charge. Sam grew up looking up to his older brother, and having him tiny wouldn’t change that a bit. It would be a lot like in Brothers Unexpected, where Sam gets adopted by Jacob’s family. Sam is much smaller than Jacob, but as the oldest, is also trusted to watch out for the kid and Jacob learns fast to listen to him (sweet tol teddy).
Not sure if we’ll ever get to writing a Brothers Together switchup, but it is a lot of fun to think about the Weechesters no matter who’s what size.