I think they wanted Miki to retain her "Petite" size being an Asian based figure and having her share the G2 Body would have made her just as tall as everyone else.  Kinda their way of remaining true to her asian origins. Miki 2.0's body is still shaped and sized the same as Miki 1, or close enough that they look the same side by side. She's just regrouped and supposedly better jointed now.