mental_out said:
wasnt everything bigger back then due to there being more oxygen in the atmosphere? Is this the solution for manlets?
Not back then if I'm not mistaken. This is still the Cenozoic, so it's not that old.
You may be thinking about the Paleozoic, more precisely the Carboniferous period, when insects were gigantic because of high levels of oxygen indeed.
I'm not sure the oxygen levels affect the size of vertebrates much.