i've not kept it a secret that Mistress is both bigger and stronger than i am. i wrote about it before here on this blog.
i'm sure there are other marriages and relationship where the Woman is physically stronger than the man but it would be fair to say they are in the minority.
Knowing She can physically impose Her will upon me is a constant reminder of my submission, Her Dominance, my inferiority and Her Superiority.
There are power dynamics at play in any D/s relationship. To me this is among one of the more powerful ones.