I agree with the above poster. After an MRI I was told I had an 8 wk size uterus and I was told I had possible Adenomyosis. That's an enlargement of the uterus or something like that. So you might want to read up on that.
Good luck!
I am not a clinician, but I believe a mobile uterus is where it moves around in your abdominal cavity. I think this is normal, meaning you want it to be mobile.
6 wk size means it was looke to be the size of a 6 wk pregnancy. It may be that you have a larger uterus than normal, not that you were pregnant. I'm not sure either is something to worry about but I would call your other dr