t’s always going to be an asymmetrical comparison.
Do you compare the Roman empire as its peak with 12th century France or just Roman Gaul? Significant parts of Europe were never under Roman control to begin with and those would have been more advanced in the early middle ages than during Roman times. Besides that many regions of Europe such as Britannia, Gaul, Germania and the Danube region had a low population during Roman times whereas they had a higher one during the Middle Ages.