If anything, I would think being bigger makes it much more difficult to attack.
No, with Bitshares because of centralization hacker only need to target delegates regardless of the size of DPoS market cap.
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.
Sure, but with Bitcoin you have to buy or manufacture the equipment and spend the time and money setting up the large mining farms , and spend electricity to fake 1-3 transactions. With PoS or DPoS all you have to do is hijack or hack the centralized keyholders without actually spending any money or resources. Who knows how many Bitshares keyholders or Nxt large shareholders are already compromised at this moment.
Delegates are still voted in by stake, stakeholders could simply vote them out instantly if they are compromised (though good luck compromising 51 de-centralized delegates), Bitshares makes this a very fast and simple process. No attacker would be dumb enough to target the Delegates. They would still target the stakes.
Doesn't really matter how many stakeholders are hacked (just like Bitcoin, if you get hacked, you lose your coin), since the hacker is now the new stakeholder, even if a hacker controls 51%. The dis-incentive to attack is built in. Why would you attack the eco-system you hold 51% stake in? and destroy its reputation and value, and in the process destroy your own wealth??? it goes against human nature. (and again, good luck hacking enough stakeholders to get 51% stake, unless big stakeholders somehow are all morons, they will make it impossible for you to hack)
Like I said, you can make up these scenarios about PoS all day long, just like I could make up these for PoW too. The difference is, your scenario doesn't actually happen in reality, and my PoW scenarios does happen.