I've been having a similar discussion with a colleague of mine. The US$2 trillion daily market is indeed that - a daily market. If you split that over all the "moments" in the day, and then split it again over all the pairs being traded, and lastly consider a more minor pair, is it not possible that US$100 million being bought/sold in one moment could move the market?
This was just a discussion we had, and I'm not convinced myself .... suppose I should look at the math a bit closer.
If everyone was trading the same "minor" pair in the same narrow timeframe during an off-peak part of the trading day, maybe you do get a move. Heck, even with a major pair you might see something, depending on the time of day. But that assumes the trading is passed through from the retail brokers (market makers) in to the general market (read inter-bank) by those brokers and not just offset by some exposure they have elsewhere in their customer position set.
Let's say we're talking about a $100m buying in AUD/NZD. In order for the overall market rate on that cross to move, all the brokers (or at least a good chunk of the transaction volume, anyway) would have to go in to the inter-bank market at the same time and buy AUD/NZD. Then the market would move - at least temporarily (if there were no follow-up buying it could very well settle back to where it started). If the inter-bank market doesn't see the order flow, though, it's not going to be impacted.
Think of it this way. If I were to sell you 1m shares of Google stock right now in a direct transaction that didn't go through the exchange, do you think it would move the market? Nope, because the market wouldn't know about it. It's the same with the retail forex traders in terms of the inter-bank market.
Now if a buy order of $100m USD/JPY came in to Citibank from GE, the market would probably feel it, at least a little, depending on the time of day and overall volume. That's because GE is playing directly in the inter-bank market through Citi's dealing desk.