User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4674269-20111219194424/@comment-1011548-20111219200014

Yep, I'd say a high oxygen concentration in the atmosphere seems like a reasonable assumption. Big active organisms also need more oxygen than smaller ones, so a high concentration of oxygen might help explain why gigantism is so prevalent amongst the ecosystems (although not everything is gigantic - IIRC there are normal birds flying above Area 6 of the F'n'H, a normal crayfish in the Old Jungle, what look like baby chickens in the Felyne Kitchens, and a turtle in the Mountain Stream).