PERFECT examples, ewest. With the Northern BG in the bottom pic - the mouth is virtually dead center top-to-bottom. With the CNBG in the top pic - the split is more like 60-40 with noticeably more CNBG above the mouth than below.
So - if I'm right CNBG have bigger helmets, Northern BG have balanced helmet and breastplate sizes. Probably due to a difference in gladiatorial styles

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If they were warmblooded, I'd think it was due to adding mass to the belly area for insulation up North. As it is - ???