Aren't "market-sized" deals measured in millions because banks are making one deal to cover lots of smaller deals that they've made with their own customers? I

Aren't "market-sized" deals measured in millions because banks are making one deal to cover lots of smaller deals that they've made with their own customers? If someone can cut out the middle-man and let the small deals trade with each other directly, there is no reason why people can't get the mid-market rate for a trade of $100 never-mind $1000. This is the idea behind currencyfair.