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.
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 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.