Without seeing your actual images, it is hard to answer this question via the internet. That being said, the measurements on the MRA and CT scans you had should be relatively comparable. That doesn't appear to be the case looking at the numbers you have written. The CT report states that your ascending thoracic aorta measurement is stable, but the numbers don't match up. I would call your cardiologist and discuss the difference between the two reports. He/she can better clarify the discrepancy.
Hope that helps!