I'm not sure if I'm allowed to talk about it in this thread, as it is more relevant to the UFO one, but duh, they had anti-gravity, obviously!
In all seriousness, it is a matter for much debate, and no one has a good answer. Self-defense is brought up a lot, thinking that they had to keep growing larger to defend against predators, and the predators kept getting bigger to eat them, etc. Or being large and cold-blooded helping with temperature regulation, but then you have people saying that most of the predators were likely warm-blooded, so that doesn't make a lot of sense either. Short answer, we have no fucking clue.