How do we know the size of the Bitcoin transactions? How do we know that the price action is not being driven by a couple (or otherwise small group) of investors that are just flipping small fractions of Bitcoin back and forth?
Exchanges' order books are usually public. See for instance https://www.gdax.com/trade/BTC-USD. The graph in the middle is a helpful visualization of depth.
I'm not going to quote any specific numbers here because they would be instantly obsolete.
Of course, there are lots of exchanges, so to get the "big picture" you'd have to aggregate their data.