Isn’t she like 5’10? Michelle’s body and height perfectly fits the image of MM.
I think they were looking for a real actress to play a challenging character not just any celebrity to dress up as Marilyn, we’ve seen that many times before. Michelle’s performance has solid Oscar nomination written all over it and now I perfectly understand why the filmmakers didn’t go for likes of Scarlett Jo, who I always thought would be an ultimate choice to play Marilyn. Its an actor’s role.
As with the differences, its impossible to achieve 100% resemblance.