I mean transaction volume - doesn't matter if it's 10mBTC or 10 BTC it's still one trade but are those trades going onto blockchain? I don't think so.
Trades aren't transactions on the blockchain. When you and everyone else sends coins to an exchange, it is just held in a wallet by the exchange and your balance is just a row of their db. When I sell you BTC, your account rises in BTC and drops in Fiat and my account drops in BTC and rises in Fiat. It's all just added and subtracted by the site's back end software. There is no need to broadcast all the trade volume.