I know that the depiction of these warriors as fearsome and large come from Julius Caesar's commentary on the Gaulic Wars, and there are other instances of ancient writers describing these warriors in a similar manner. One /r/AskHistorians post I read stated that one writer felt the need to illustrate when a Roman soldier defeated a "barbarian" in a one on one fight, indicating that it was an impressive feat, possibly due to the size discrepancy. While Caesar had an ulterior motive to depicting them this way, the ancient writers could be reasonably believed as accurate.
However, a post in /r/HistoricalWhatIf asking about how a modern NFL player would compare physically to ancient soldiers contained a popular comment stating that these barbarian soldiers were, on average, no more physically imposing than a typical Roman. The poster states that this idea comes from Romanticized paintings from the 18th century and is akin to depictions of Vikings wearing winged helms and other historical cliches. They made this claim based off of the assumption that since both cultures we're agrarian, an agrarian diet would lead to similar statures. Below is said post.
Who is right? Were these Gaullic and Germanic peoples really physically larger than a typical Roman? I obviously trust /r/AskHistorians a great deal more given the general expertise of the posters here, but I would love some clarification on this.
Not to discourage any newer answers but there's some good answers in this older thread that you can check out: