Someone is moving some serious coins around, I
If I buy a piece of penny candy and pay with a $100 bill, then repeat doing so every minute for ten hours straight, the cash transaction log will show just under $120K of cash changing hands even though only $6.00 worth of candy was actually sold (assuming there is still such a thing as a piece of candy that costs just a penny.)
Mixing has this same property. To mix away a few BTC there might be coins that are hundreds of thousands of BTC in size, being chipped away a few BTC at a time.
But there are all types of areas that bitcoins can start to displace other types of large financial transactions.
Some businesses have no access to credit and are entirely dependent on customer cash flows. Because bitcoin transactions are lightning fast compared to traditional bank wire transfers, those businesses will start wanting to receive payment in bitcoin. If work routinely stops because the needed money is tied up working its way through the banking system (which happens all to often, especially for flows that cross borders), Bitcoin helps to cut down dramatically on that idle time.
Another scenario might be for auctions, such as those for property. Cash is risky to carry and inconvenient so the method that works is for there to be a requirement that bidders carry a cashier's check for some minimum amount (e.g., $5K USD for home sales). That requirement increases the chances that the ultimate winning bid will truly carry through with full payment. But obtaining cashier's checks is inconvenient and expensive as well -- especially for bidders who aren't regulars or don't necessarily plan to bid aggressively. Bitcoin can serve as a convenient method to secure the winning bid while the winning bidder then goes to obtain a cashier's check for the full payment amount.
There will be a lot of scenarios where lots of bitcoins change hands and there is little visibility into why.
