First off, I understand pitting the Roman Empire of the 1st-4th centuries vs the Franks in the 6th-9th centuries to be rather a disingenuous comparison; however after finishing reading "The Fall of Rome" by Ward-Perkins, who argues that nothing came close to the level of civilization or prowess that the Roman Empire had, for a very long time in Europe, and yet, the Franks were able, especially under Charlemagne, to create an Empire, that stretched all the way to Poland. A feat, which had never been done before by the Romans.
I don't buy the idea that because Franks were "Barbarians" and the Romans were "civilized" meant that the Franks had an easier time conquering and assimilating the various peoples to the East of the Rhine; but you then have to wonder what the Franks did right, which the Romans failed to do.
Was it just because the Romans after Teutoburg Forest had no incentive to invade Germania again? But then why would Charlemagne want to, instead of conquering the far richer and more developed Visigothic Spain for that matter (especially after he subjugated the Lombards in Italy)?
For a very long time (between ca. 400 and 800), the Franks had no interest in expansion east of the Rhine. Charlemagne only managed to do so after a very long campaign, and even then it can be argued that the conquest was not very long-lasting, because the descendants of the local Saxon aristocracy took over the eastern part of the empire for themselves during the Ottonian dynasty (and Bavaria was basically ruled by a proxy/vassal relationship rather than directly conquered). Until Charlemagne, the Franks suffered constant troubles on the Germanic border, just as the Romans had done. What Charlemagne did differently from his Roman predecessors was taking Vegetius' advice on fortress-building to heart. After the initial expedition (as the Romans regularly did), Charlemagne installed Frankish aristocrats and had them build fortresses at strategic locations, both to interrupt Saxon armies from moving about in an organised way, and to reinforce the infrastructure for future Frankish campaigns. This must, of course, have been extremely costly. But while the Romans had a standing army and had to pay wages for their soldiers (which made campaigns and big building projects expensive), the Frankish military structure was itself sustained by campaigning. The bulk of the army would have been obliged to serve every year for the campaigning season, while the court was held together by loot gained on campaign (rather than things like taxes, which sustained the Romans). This is why Charlemagne had an incentive to expand, even in relatively marginal territories like Germania. For Charlemagne, the campaigns basically paid themselves, in the first place by defeating enemy aristocrats in the field and taking their mobile wealth (gold, weapons etc.), next in slaves, and thirdly in extremely oppressive taxes in agricultural products (one third of Saxony's cattle was carried off over the duration of the wars) in a systematic attempt to bleed Saxony dry.