There is a valley near Anchorage, Alaska where the vegetables grow to ridiculous size during the summer months due to the Sun being up most of the time. I suppose if dragonflies lived in a totally oxygen saturated higher pressure flight regime, the deterministic DNA programming for growth might produce larger specimens than we see today. Or the sudden loss of the canopy could explain why creatures with expensive energy requirements in our current atmosphere went extinct?
By the way, large dragonflies were the hardest to catch, ranking up there with the elusive Zebra Swallowtail.