no one yet, has been able to say for sure why animals were so large in previous time. yes the pure air theory is one of them. another is you have to be big to eat big animals and big plants and by eating big animals and big plants that, in turn, makes you big. there are a couple of more.....read into them. they all make valid points but all have problems with them.

there was a recent finding of a giant prehistoric snake. i can't remember how long it was but it was crazy long.