If anything, I would think being bigger makes it much more difficult to attack.
No, the centralized nature of bitshares allows a hacker to 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.