Here is a revealing article on well-being in pre-historic and historic Europe. It uses estimates of human height as the proxy for how well people were doing. The main reason for taller average height across these time periods is assumed to be better nutrition, lower infectious disease load, etc. For anyone who's ever had experience eating little animal products vs. a lot of them, it's clear that better nutrition isn't good only for the body but for the mind too. Same goes for being more free of pathogens.
It's an interesting read, but you can skip to page 42 of the PDF to see the average heights plotted over time, from the 8th century B.C. through the 18th century A.D. (i.e., before the industrial revolution, when we were still in a Malthusian world), and for three major regions of Europe -- Mediterranean, Central-Western, and North-Eastern.
During the Roman Kingdom and Republic, their people averaged about 5'6.3", and once the Empire's days were numbered, they were also about 5'6.5". However, at best during the Empire they were one inch shorter, about 5'5.6", and during the 1st century B.C. -- the time of Julius Caesar -- they were just 5'4.6". A good deal of this Imperial shrinkage appears to be caused by their curtailing of herding cattle, and thus losing meat and milk, in favor of cultivating wheat (see PDF page 40). This is just another example, albeit from historical times, of the devastation to our health when we give up animal products and make it fashionable to chew on grass instead.
The Celtic, Germanic, and Slavic tribes and their descendants weren't so decadent, so over the whole time period their average height is somewhere around 5'6.5" and 5'7" -- not as tall as today when disease is less rampant, but still pretty good for pre-antibiotics people. How muscular they were does not show up in the height estimates, but we can infer from how much meat and milk they ate, as well as from first-hand descriptions, that the average young adult male would have looked like The Dying Gaul (complete with rich rock singer hair, in contrast to the bald Roman grain-munchers).
To get a better feel for just how terrified the invading Romans must have felt before the taller Barbarians, let's run through some numbers. In the 1st century B.C., the average Roman was 5'4.6", and the Barbarians about 5'6.3". What percent of each population would stand at least 6' tall? (I'm assuming a standard deviation of 3 inches that we see today.) Among the civilized Romans, only 0.68% -- not even 1% -- whereas among the Barbarians, 2.87% would have, or over 4 times as many per capita.
Now, 3% may not sound like much, but that's only assuming you picked Barbarians at random. Those who made it into the ranks of warriors were more elite. Let's say they were the top 10% of their group by height -- then their minimum height would be 5'10.1". Since the Roman invaders relied more on strength in numbers than on prepossessing individual stature, they probably weren't chosen to be so much taller than the average citizen -- maybe 5'7" on average? Then these Roman grunts of, let's say, 5'7" confront a warrior class who are all taller than 5'10" -- they must have needed nerves of steel to get through the Gallic Wars.
So here's another example of how much better Barbarian life was compared to Imperial Roman life. They were taller, probably fitter, and certainly less neurotic since they weren't so stung by the pangs of hunger that a grain-heavy diet causes, not to mention breathing more freely (quite literally) on account of lower urbanization and thus lower rates of crowd diseases.