User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-3481504-20130307031256/@comment-5767263-20130308053247

"What I'm saying is, they simply could not survive in the void of nothingness as God and Death can. Its pull would rip their very being (existence) apart."

"Oh yes, they can survive in space - space is made up of matter and molecules -, but the void of nothingness, as it's name implies, has nothing it in."

If the void has nothing in it, then there would be nothing to pull or act on the leviathans. A true void is literal emptiness - there's no reason why the leviathans wouldn't be able to survive in it, because they don't need heat or air to survive and there would be nothing to act upon them.

"Too much for them to chomp? You're joking, right? They're the things that wolf down everything else--by everything, I mean: Angels/Seraphs/Cherubs, Demons, Alphas, Deities, monsters, and of course, humans. Castiel inferred that they hold the potential to essentially devour the Universe."

No, I'm not joking. Since nobody has any idea how big Heaven is or how many leviathans there actually are. It may simply have been so big that the leviathans didn't eat all that much of it before God decided that they had to go. It was said that they had the potential to eat everything in creation and they very well might, but once they're in Purgatory, they can't really access "creation" anymore and thus, the threat is negated.