But Zexion is dead. If that was the case, the others who died in CoM should have lesser nobodies.
And Xion isn't technically dead.
Then by your logic, wouldn't that mean the only nobodies left over after the deaths would be the Dusks and Sorcerers? No, the nobodies that took after the Org members still exist even after their deaths, which leads us into SA's statement from earlier.
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.