I really don't think time spent in the Organization has to do with yes/no having lesser Nobodies to control. Way I see it, Roxas was supposed to be a Samurai, but because of the strength of Sora's Heart, Roxas ended up retaining his human form.
His association to that specific type of Nobody still existed, however, thus making for his control over them - he truly is the strongest Nobody of that type.
It ties in with Saix and the Berserkers, as well, Xaldin and the Dragons, Xigbar and his Nobodies, etc.
Xion having no type of Nobody to rule over - unless they'll introduce a new type of Nobody for her to control in Days that for some reason died out before we got to see it in KH2 - might just tie in with her being not a Nobody, or alternatively, being that messed up a Nobody.
Or a third option - seeing how as far as we know, her weapon is the Keyblade, I think that would associate her to the same "Class" as Roxas - being, for the case in point, a Samurai.
That's just an example but the principle still hold - should Xion be associated to a type of Nobody another Member of the Organization already has control over, and that Member was stronger than Xion - then she'd have an association, but wouldn't be the Lord of that class of Nobodies.
Roxas = Stronger than Xion = Lord of the Samurais, with Xion as his "right hand man" or something.