If you, a single person, owns all bitcoin, noone would buy a single satoshi from you.
Happy? Do you miss the forest for the trees a lot, or only when you're on HN?
In my mind, it's conceptually similar to elastic/inelastic demand curves.
This is why institutional traders have to break up buys or sells over many transactions. If you put up a huge block saying "I want to buy/sell $X million / Y million units of instrument Z" it is going to strongly drive the market away from your bid/ask. In a market like BTC, it might drive the price down 50% within minutes.
In other words, who is going to be on the other side of your market? If you are selling 1000 BTC, and if the top bid price has a quantity of 0.1 BTC, you will blow through the first level, then the second, thrid, etc, through dozens of decreasing price levels, if you are doing market orders. You will take out the ENTIRE BUY SIDE OF THE MARKET, and the price of your last transaction will be thousands of dollars lower than the first price. If you are doing a limit order, then you provide the lowest price you are willing to go, so you might only be able to sell a few coins.