The basic question "Why planets revolve around the sun?", is explained by taking into account the heaviness of Sun.
No, the basic question that science has been able to answer is : How planets revolve around the sun, not Why.
This means that a mathematical model of assuming heavier masses attract more led to the central force of gravity formula of Neutonian gravity:
The two masses contribute proportionately to their value.
The observation was that planets are ordered about the sun according to their masses and Neutonian gravitation showed us how we could model their motion and predict motions of other celestial bodies using the proposed theory.
So the theory can answer why the earth has a large radius orbit around the center of mass of the solar system and the sun a small one, but in the end the why large masses contribute more than small ones to the force between two masses. ends up to the answer "because that is what our observations have told us"