The German population was definitely growing during Roman times. We see Germans running out of room in Scandinavia and the area immediately around it and expanding at the expense of Celtic tribes in what is now Germany and expanding east into what is now Poland and finally the Goths migrate out of the Baltic to Sarmatia before invading the Roman Empire. So a) they were obviously able to accomplish this and b) the carrying capacity of the German Heimat during classical times was easily exceeded because of the agricutural technology then available, neccesitating expansion and c) the Venedae Slavs were sufficiently numerous and advanced enough to where the Germans couldn't just push them farther and farther east.I may not us it as a paper, doing something ells, I may use it some other time.
Anyways, but the thing is the German population was not as small as one thinks for the time that is. Yes, it was not as big as it is now or even the Medieval era, however it was not just a few people in the woods ether. But one combined force is a pretty big number of people. It also seems that they fought well and well enough to catch the Roman eyes.
The thing about the Xiongnu is that their tactics will not do nay good in a place like Western Europe, there hills, alps, a bit rocky, trees all over and there are even such thick forests that even modern armies had to go around them. The Xiongnu will ether say at the steppe parts of Europe or adapted the Roman or German way of war or just combined them. But even than it seems what was left of the Huns in Europe faled to adapted and were defeated by Germanic tribes
Keep in mined in Roman's strongest the Roman military did well against steppe attics using heavy infantry and combined arms, the Romans were very, very good in adapting.
If most of the German tribes were in China than the same thing would happen. Ether they would adapt a Chinese or steppe way of warfare or stay at the rocky and heavy forest parts in Asia were they can fight very well at, or maybe combined them adding their own way of war to the mix. Keep in mined the Germanics tended to be big people (even today).
Ancient China was not one unified nation yet and some times had one unified Empire yes, but it was the smaller Waring stats that helped expand China as well or maybe more so.
Ummm, most of the Germanics were not exactly "sedentary" ether they did move around a lot too. I think the Romans were more worried about other Empires near them, but in the sometime there is nothing to take from the Germanic people ether and they did what you describe the "nomadic empires". Keep in mined that most of the Germanic people were not all like what is almost always portray in popular media, the Germanic people were much more advance than the Zulus. There was a nice amount of arms & armors that the very Romans adapted, and the Germanic did fight fearlessly and pretty well even to the Romans.
Most (not all) of the Germanic people were like a sami-nomadic people them selves that often moved around to place to place.
Which did not put them in the same category as Motu's Xiongnu.
I agree that the Xiongnu would be at a disadvantage in Western Europe (unless they could reach the Iberian Meseta) since they proved to be so under Attila. But Gaul was underpopulated compared to Italy and Greece. If the Xiongnu (or the Kushan Yezhi) had taken Rome, likely Gaul and Brittania would have regained their independence.