Let us pass by your somewhat hyperbolic comments about Roman vs. German height.
The Romans typically dominated the Germans in combat from the time of Marius (100 BC) till well into the 4th century. That's about 500 years during which the Romans generally, though not always, won the battles and the wars.
Rome collapsed because it disintegrated internally, not because it was conquered from the outside by the Germans, who merely took advantage of the Empire's weakness to grab what they could.
In one battle in 9 AD the Roman Army lost 10% of its strength (of the global Roman Army); the fate of those troops wasn’t known for two years until troops again reached that area. There are no Roman ruins scattered throughout most of Germany; like Scotland they despaired of ever winning there. Winning border skirmishes that ground down Roman strength wasn’t a viable long-term strategy.
The height disparity between the two is well documented; I belong to neither group so have no horse in this race.