This doesn't sound right to me. Fender has been making different neck profiles for years, yielding varying shapes, widths, and thicknesses, and the "Posiflex" system to my knowledge can be embedded into any of them.The neck of the 2011 MIA has graphite rods in it and this makes the neck thicker. The MIM does not and therefore it feels easier for my hand to connect to and feel one with.
Maybe someone will correct me on that, but I doubt they are the reason why your particular MIA neck was slightly thicker than your MiM neck.
Again, not true in general. My MiM jazz is nearly 10 pounds, while my MiA is about 8.75.The MIM is also slightly lighter than the 2011 MIA .