Dominance is about personality.

Yes, it is nice to also be physically strong and able to let your sub "feel" that strength, but I thrill at what my girl does because she wishes to please me. That is all about the person I am and not the body I have.

Case in point... your question, by its very nature, would dismiss 99% of the domme-malesub relationships if D/s were about physical strength.

And for those who do revel in submitting to a physically stronger person... and those who revel in dominating a physically weaker person... the thrill comes from knowing you can... and that's mental too.

The only relationships that are based solely on physical dynamics are nc relationships... and they mostly exist in fiction... ooops... that's mental too.