The "big one" could have been one from MtGox for example.
OK, that is probably a MtGox-address, judging from the amounts and how often they come in.
Nevertheless the total BTC volume of the other large addresses would be in the hundreds of thousands and would likely exceed the total number of deposits at BTCST especially considering that they aren't that old. And they are all immediately following, ie. next step, addresses.
I would be interested in seeing a Deposit address or ten, to see what happens with deposited funds. As far as GPUMax, based on what I've seen, there is either nothing of note going on there (I'd need deposit addresses to know more), or this is a massive money-laundering operation. Money launders can make upwards of 50% of what flows though them, so that would both pay for the whole show as well as explain why Pirate doesn't feel like using his own BTC.
I am quite qualified to do blockchain analysis, and I promise not to reveal addresses sent to me, just the information that I find.