There are whale volume discounts available on almost every exchange. Mid-level whales can get 0% rate for trading over certain number of Bitcoins. Back in 2013-14 you could even get NEGATIVE rates for trading large amounts. I.e., if I trade a certain number of BTC, I get a rebate at the end of the month. High volume boosts exchange's rating and brings more customers, so it is a no brainer to offer rebates.
If you remember early days of Chinese exchanges, they were doing millions of coins a day in first week of their launch. That was clear example of Exchanges themselves doing wash trading. All volume data is available in public domain for verification.