Gaven's response was basically "they aren't big enough yet". I can't really see the logic behind "being bigger makes it easier to attack". Tiny PoW altcoins gets 51% attacked all the time, even sizable PoW like Dogecoin gets 51% attacked several times, while the biggest ones Bitcoin and Litecoin never got attacked.
If anything, I would think being bigger makes it much more difficult to attack.
PoW alts are very vulnerable because any mining pool or large miner can easily create a 51% attack for a brief moment and than point their resources miners back to securing litecoin or bitcoin .
PoS can be attacked by either covertly leasing, buying or hijacking the 50% of the original stakeholders assets. This takes time and an organized effort, thus the bigger these currencies get the more incentive their is to fund such an effort.
Except, in PoW, you don't even need to lease/buy/hijack 50%, you need roughly 10% of Bitcoin marketcap value to buy enough hardware to 51% attack Bitcoin. Basically if you could simultaneously hack discus fish and ghash.io, you could 51% attack Bitcoin today, right now.
PoS CAN be attacked, of course I agree, but it's still much more difficult to attack than PoW, therefore it's a better alternative to PoW.
Buying 51% stakes in a PoS will cost you astronomical amount of resources, while buying hardware to 51% a PoW is easy and won't push up price.
I have already explained all of these points earlier in this tread.
Again, you have to question why these attack theories doesn't happen to PoS altcoins in reality, but does happen to PoW altcoins. "Not big enough yet" is not the answer, I assure you.