Justification for large arthropods (biome of the coal-bearing stage)

A carbon forest biome could be added, consisting of swamp forest with large vegetation, sigillaria-type trees, Lepidodendron and Calamite. Alongside large plants and as mods giant arthropods like those that existed in the Carboniferous stage (Arthropleura, Hylonomus) and the occasional reptile. While it would vaguely justify the existence of large arthropods, it would also raise the question of how a human can live in conditions with that amount of oxygen.