Prince Wingfire loves to be large and in charge -- not as a bully but as a playful guardian. His wife, always happy to please, loves to let him live his fantasies with her. Some of his most special moments are when she exists to be his little toy to love and pleasure him.
In turn, Foxfire has similar fantasies with her husband. So the two take turns being the little one. (They never have to worry about stepping on each other by mistake; the size-changing magic takes care of that.)