I think the novel says that Michael isn't reducing the size of their regimes, he's just not permitting them to recruit more men. And he's secretly letting Rocco build his own regime.
I believe Michael was not only feigning weakness to flush Barzini out into the open--he was also testing Tessio's and Clemenza's loyalty. He admonishes them to stick with him for a year, then they can either go with him to Nevada or form their own families. Obviously his "weakness test" worked in both cases.