it looks like there's a rough correlation (green lines) with the production of blocks.
but how does a 50 BTC block correlate with a wallet transaction of 9,000 - 75,000 BTC?
Block creation corresponds with transaction confirmations, so it's possible that these assumed traders did swaps of one amount, divided up into 8 or more transactions, then waited for confirmation before continuing on the next set of swaps. I don't know why the sizes are as such, but it's possible that either it's a rounding thing, a cumulative effect of combining wallets before making a transfer, or just an artifact of how bitcoinmonitor.com displays transfers when multiple addresses are involved.
What I mean by "rounding" is as in the following: Say you want to sell 1,987,654 bitcoins. You and the buyer agree to do this with cash or some equivalent in 10 lots of 198,765 bitcoins and then wait for confirmation before continuing. But, rather than send them in one big chunk, you subdivide into round numbers and send transactions of 765, 1000x8, 10,000x9, and 100,000. Once those transactions are confirmed, you start over.
Why do this? Because when trading large amounts of small denominations, it's better to subdivide in case there's an error or some party tries to rip off the other. That way, you're only risking the lot size at any one time. The transaction sizes are just for convenience and ease of bookkeeping, like when you're counting cash in a till and you tally the numbers of each bill size separately.
What I mean by a "cumulative effect" is that it's possible that there was a few trades for, say, 10,000 and the recipient is receiving at multiple addresses and then consolidating these smaller trades at one address. In other words, the trade might have just been in lots of 80,000, but the buyer got 10k BTC, 10K BTC again at another address, and sent this to the first address. Now he has 20k in one spot. The same thing happens when with the next 20k BTC: 3 transactions appear for 10k. This happens until the buyer has all 80k. Then he consolidates these 20k wallets into 40k wallets. And two 40k wallets into one 80k wallet. Then a new block is found and he has confirmation on all these, so he and the seller start on the next lot.
In other words, even though we're seeing like 200k BTC in transfers for each block, it's really just the same 80k.
Or it's a dude who had 50 different wallets for safekeeping, following the "don't put your eggs in one basket" motto, and he's combining his wealth in order to make a trade.
Well, that's all just me guessing, but I hope someone more knowledgeable can come in here and set me straight.