Are you sure about that? It seems just the opposite of EU that less surface area to mass ratio would create more repulsion... Can someone smarter in EU than I enlighten me on this?
I think I agree with Touserman that the angular momentum combo determines or situates the orbit.
Miles Mathis gives his explanation how the orbital distance can be calculated from the sizes and densities of the planet and the sun here:
A Mathematical Explanation of the Orbital Distance of Mercury
Although it is mostly dependent on the sun's fields, from what I derive from the article the rule would be:
Increased mass/same size (=increased density) => slightly greater orbital distance due to extra repulsion
Increased mass/same density => identical orbital distance
Decreased mass/same size (=decreased density) => slightly closer orbital distance