The Celts and Germans ate much better than the average Roman. The Germans during Arminius’ period were up to a foot taller than the Romans. Some of that is genetic but most of it was better diet. The average Italian was a peasant who ate grain, grain and more grain, like most peasants. The Germans (and Celts) were at least as much pastoralists as agriculturalists and they are much more dairy and meat.

The only groups of people that often see stunting in growth are farmers at the Malthusian margin. Hunter gatherers and pastoralists either starve or kill each other quickly enough that you don’t see the kind of years long periods of insufficient calories that don’t actually kill you that are common in peasant societies. Pastoralists always and everywhere raid, skirmish and steal each other’s livestock. They’re preadapted to warfare because of mobility and once they get horses basically all adult men are cavalry, in cases like the Huns, Mongols and Scythians. Even people without too many horses but who live and die by cattle are far more mobile than grain farmers.