Yes you are correct about the differring fighting styles but that didn't count for everything. There were instances where Romans fought one-on-one duels with celtic champions and won (though participation in these was eventually outlawed by the Republic)
Also your use of "Celt", "German" and "Roman" seems to be ethnic. Celtic is not an ethnic term by any means. The lands that housed celtic speakers had many different ethnicities that used celtic language and customs. So how can one speak of Celts being shorter than Germans or taller than Romans? There were big men in their ranks sure, but what culture doesn't have their brutes?


Do tell about this . That tells people had huge cajones back then if they left the comforts of line combat to go one on one with a brute.