I've been listening to Mike Duncan's history of Rome and in it he mentions how the Roman army became increasingly Germanised after the reign of Constantine. The reason he states is that the Landowners actively hid fit freemen so that they had men to work their estates which meant that Rome was forced to look to the Germans for men. Aurelian even started to settle Germanic tribes inside the Empire to increase the tax revenue.

He also talks about how disastrous the battle of Adrianopole was and how much difficulty Theodosius run into trying to replenish these loses.

During the second punic war Rome lost something like 25% of its Male population and still won the war. They lost around 80 000 men at Cannae and were able to replenish those losses when they only controlled Italy and not the entirety of the Mediterranean.

How were the Roman's able to replace losses so well in the early period but struggle to replace 20 000 after Adrianopole even with their vast empire? Was it caused by the antonine plague and subsequent wars of the third century? Did people simply have less kids? Or was it something else?