Size of embryo vs size of sac?

Amy
Okay so I just came back from my Doc. appointment and not feeling as excited as I should.  
He did a vaginal scan and the first thing he said was that things look a little "irregular" in that the size of the embryo is really small compared to the size of the sac.  
Then he did the measurements and that seemed to change his mind.  The embryo is measuring at 6 weeks and my sac is measuring at 7 weeks.  He says he is happy with that. 
There was also a strong heartbeat that he could see and hear clearly. 
He put me on progesterone tablets for the next 6 weeks and wants me to come back in two weeks for another scan.
I had a MMC in March so I'm really scared. 
Thoughts?  Anyone had something similar happen?  Are the sac and embryo supposed to be the same size?