The people who have done this have immediately been caught out by the Bitcoin community, the thing is, bitcoins have a hard limit of 21 million, if exchanges ever attempted to rig their trade volume to something unrealistic they would immediately get caught, especially if they're dumb enough to post up their numbers.
Be assured that people have already thought of this and are keeping an eye on things, there will be exchanges and companies that certainly try don't get me wrong, but they are very often found out, this is partly why I'm so skeptical of the Bitcoin ETF people keep ranting about for example.
How can people know if bitstamp is operating on let's say 2% fractional reserve, if they refuse to audit?
If bitstamp operate with 102% of the BTC they currently have (" only" 2% fractional reserve) they can use a
monetary multiplier of 2x, so they CAN double the money. They don't need to cash out all this money, i suppose they CAN'T cashout bitcoin, but they can in fiat, if they suppose a small fraction of deposited btc will remain offchain.
There won't be exact figures obviously, but it's easy to estimate, for instance, with mt.gox the price was going stupidly high thanks to the now nicknamed 'willy bot' ( Think I got that right ) and a bunch of people had worked out that Bitcoin's price went high because of that trading bot bumping up the price rather than it being because of any legitimate trade volume.
Also, don't forget about the blockchain, even if a company tries hard to hide it's tracks, currency can still flow from one address to another, so in the end it's just a matter of time before it's tracked down.
